oauth2: Add token_expire_grace_secs setting

Author: sirainen

docs/core/config/auth/databases/oauth2.md

@@ -299,6 +299,11 @@ report OIDC configuration URL as `openid-configuration` element in error JSON.
 ## Timeout in milliseconds
 # timeout_msecs = 0
 
+## Specifies how long after token expiration it is still accepted. It
+## shouldn't be 0 because clocks aren't exact, and network can add some
+## delays. v3.0.3+
+# token_expire_grace_secs = 60
+
 ## Enable debug logging
 # debug = no