Add `graphql_json` logfile documentation

doc/administration/logs.md

@@ -288,6 +288,19 @@ installations from source.
 
 It logs information whenever [Rack Attack] registers an abusive request.
 
+## `graphql_json.log`
+
+Introduced in Gitlab 12.0.
+This file lives in `/var/log/gitlab/gitlab-rails/graphql_json.log` for
+Omnibus GitLab packages or in `/home/git/gitlab/log/graphql_json.log` for
+installations from source.
+
+GraphQL queries are recorded here. For example:
+
+```json
+{"query_string":"query IntrospectionQuery{__schema {queryType { name },mutationType { name }}}...(etc)","variables":{"a":1,"b":2},"complexity":181,"depth":1,"duration":"7ms"}
+```
+
 ## Reconfigure Logs
 
 Reconfigure log files live in `/var/log/gitlab/reconfigure` for Omnibus GitLab