docs: clarify 64 KiB response-body buffer in authz plugin docs

Adds a "Response body size and partial buffering" subsection to
docs/extend/plugins_authorization.md documenting the 64 KiB
maxBufferSize constant in the daemon's internal responseModifier
(pkg/authorization/response.go in moby/moby) and the practical
implications for plugins that use ResponseBody inspection.

The existing docs (lines 81-87) say streaming endpoints such as
logs and events send only the HTTP request to plugins, but don't
explain the underlying mechanism. Plugin authors building
response-body redaction or content-filtering can be surprised when
the same effect happens on non-listed endpoints whose response is
produced through multiple writes exceeding the buffer or via an
io.WriteFlusher.

The 64 KiB buffer is observable from the public moby source, so
this PR is documentation catching up to existing behavior — not
a contract change.

Signed-off-by: Matteo Panzeri <matteo1782@gmail.com>