docs: improve docs for APISIX's gRPC support

### Current State

The current docs does not have a lot of documentation of APISIX's gRPC support leading to users having to ask questions. @kingluo has documented some notes in a gist: https://gist.github.com/kingluo/65a247c302dd2e518fd3408c70241a77

### Desired State

The notes in this gist should be added to the APISIX documentation. It needs to be decided where and how these should be added.

Read the linked gist for the gRPC notes, then review the APISIX documentation to decide where they belong; the issue does not name a target page. Done means the relevant notes are incorporated into the documentation in an appropriate place.

Written by the indexing model from the issue text.