GitLab

Document that if either src implements the WriteTo interface
or if dst implements the ReaderFrom interface, then
buf will not be used.

Fixes #32276

Change-Id: Id0a69c90e255e694e7ec9f79ffe4d8391441e59e
GitHub-Last-Rev: 750e7e86d5d9b985fae7f2329fd219cacf72a62b
GitHub-Pull-Request: golang/go#32279
Reviewed-on: https://go-review.googlesource.com/c/go/+/179137

Reviewed-by: Emmanuel Odeke <emm.odeke@gmail.com>