Merge branch 'docs-dz-update-api-content-type' into 'master'

Add API content type notice to the docs

See merge request gitlab-org/gitlab!53231

doc/api/README.md

@@ -744,3 +744,15 @@ For administrator documentation on rate limit settings, see
 [Rate limits](../security/rate_limits.md). To find the settings that are
 specifically used by GitLab.com, see
 [GitLab.com-specific rate limits](../user/gitlab_com/index.md#gitlabcom-specific-rate-limits).
+
+## Content type
+
+The GitLab API supports the `application/json` content type by default, though
+some API endpoints also support `text/plain`.
+
+In [GitLab 13.9 and later](https://gitlab.com/gitlab-org/gitlab/-/issues/250342),
+API endpoints do not support `text/plain` by default, unless it's explicitly documented.
+
+This change is deployed behind the `:api_always_use_application_json` [feature flag](../user/feature_flags.md),
+enabled by default. On GitLab self-managed instances, GitLab administrators can choose
+to [disable it](../administration/feature_flags.md). **(FREE SELF)**