Document the new API extension of /api/v1/instance

CHANGELOG.md

@@ -17,6 +17,7 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/).
   <summary>API Changes</summary>
 
 - **Breaking:** Emoji API: changed methods and renamed routes.
+- Added `pleroma.metadata.fields_limits` to /api/v1/instance
 </details>
 
 <details>

docs/API/differences_in_mastoapi_responses.md

@@ -228,6 +228,7 @@ Has theses additional parameters (which are the same as in Pleroma-API):
 - `background_image`: A background image that frontends can use
 - `pleroma.metadata.features`: A list of supported features
 - `pleroma.metadata.federation`: The federation restrictions of this instance
+- `pleroma.metadata.fields_limits`: A list of values detailing the length and count limitation for various instance-configurable fields.
 - `vapid_public_key`: The public key needed for push messages
 
 ## Markers