Add api content type notice

Mention that plain text format will be no longer supported for a
majority of API calls. The original support was a necessary evil as a
result of library issues. Now when those are reserved we can remove the
undocumented and undesired behavior.
Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

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)**