docs(proxy): Clarify PROXY_OIDC_USERINFO_CACHE_TTL value

Try to make it more precise when that value is actually relevant.

Closes: #2252
This commit is contained in:
Internal Server Error - Gitea: Git with a cup of tea
500 Internal Server Error

Gitea Version: 1.28.0+dev-477-g8b6ad49a5f