We currently don't document these. Probably should; I'll put it on the list.
Until then, you can find the indexed fields by querying elasticsearch:
curl -XGET http://elasticsearchhost:port/fusionauth_user/_mapping
This will return something like the below JSON. This will vary based on your FusionAuth installation, however:
  "fusionauth_user": {
    "mappings": {
      "_source": {
        "enabled": false
      },
      "properties": {
        "active": {
          "type": "boolean"
        },
        "birthDate": {
          "type": "date"
        },
        "breachedPasswordLastCheckedInstant": {
          "type": "long"
        },
        "breachedPasswordStatus": {
          "type": "text",
          "fields": {
            "keyword": {
              "type": "keyword",
              "ignore_above": 256
            }
          }
        },
        "data": { 
        },
        "email": {
          "type": "text",
          "analyzer": "exact_lower",
          "fielddata": true
        },
        "fullName": {
          "type": "text",
          "fielddata": true
        },
        "id": {
          "type": "keyword"
        },
        "insertInstant": {
          "type": "date"
        },
        "lastLoginInstant": {
          "type": "date"
        },
        "lastUpdateInstant": {
          "type": "date"
        },
        "login": {
          "type": "keyword"
        },
        "memberships": {
          "type": "nested",
          "include_in_parent": true,
          "properties": {
            "data": {
              "type": "object"
            },
            "groupId": {
              "type": "keyword"
            },
            "id": {
              "type": "keyword"
            },
            "insertInstant": {
              "type": "date"
            },
            "userId": {
              "type": "keyword"
            }
          }
        },
        "mobilePhone": {
          "type": "text",
          "fields": {
            "keyword": {
              "type": "keyword",
              "ignore_above": 256
            }
          }
        },
        "preferredLanguages": {
          "type": "text",
          "fields": {
            "keyword": {
              "type": "keyword",
              "ignore_above": 256
            }
          }
        },
        "registrations": {
          "type": "nested",
          "include_in_parent": true,
          "properties": {
            "applicationId": {
              "type": "keyword"
            },
            "authenticationToken": {
              "type": "text",
              "fields": {
                "keyword": {
                  "type": "keyword",
                  "ignore_above": 256
                }
              }
            },
            "data": {
            },
            "id": {
              "type": "keyword"
            },
            "insertInstant": {
              "type": "date"
            },
            "lastLoginInstant": {
              "type": "date"
            },
            "lastUpdateInstant": {
              "type": "date"
            },
            "preferredLanguages": {
              "type": "text",
              "fields": {
                "keyword": {
                  "type": "keyword",
                  "ignore_above": 256
                }
              }
            },
            "roles": {
              "type": "keyword"
            },
            "tokens": {
              "properties": {
                "5ccff761-f809-49c7-a58c-27a3cb9ab650": {
                  "type": "text",
                  "fields": {
                    "keyword": {
                      "type": "keyword",
                      "ignore_above": 256
                    }
                  }
                },
                "84ef0274-70db-44da-8762-aa6bcfbd2981": {
                  "type": "text",
                  "fields": {
                    "keyword": {
                      "type": "keyword",
                      "ignore_above": 256
                    }
                  }
                },
                "Google": {
                  "type": "text",
                  "fields": {
                    "keyword": {
                      "type": "keyword",
                      "ignore_above": 256
                    }
                  }
                }
              }
            },
            "username": {
              "type": "text",
              "fields": {
                "keyword": {
                  "type": "keyword",
                  "ignore_above": 256
                }
              }
            },
            "usernameStatus": {
              "type": "text",
              "fields": {
                "keyword": {
                  "type": "keyword",
                  "ignore_above": 256
                }
              }
            },
            "verified": {
              "type": "boolean"
            }
          }
        },
        "tenantId": {
          "type": "keyword"
        },
        "timezone": {
          "type": "text",
          "fields": {
            "keyword": {
              "type": "keyword",
              "ignore_above": 256
            }
          }
        },
        "username": {
          "type": "text",
          "fielddata": true
        },
        "verified": {
          "type": "boolean"
        }
      }
    }
  }
}