doc/go1.14: document overlapping interfaces change (update release notes)
doc/go1.14: document overlapping interfaces change (update release notes)

Updates #6977.
Updates #36878.

Change-Id: I40594be85ee0a0d4b35bacc90104568d2b8a4761
Reviewed-on: https://go-review.googlesource.com/c/go/+/216997
Reviewed-by: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>

akutz/go,golang/go,nawawi/go,golang/go,cloudflare/go,CAFxX/go,CAFxX/go,cloudflare/go,akutz/go,golang/go,akutz/go,nawawi/go,CAFxX/go,CAFxX/go,CAFxX/go,nawawi/go,golang/go,CAFxX/go,nawawi/go,nawawi/go,golang/go,akutz/go,cloudflare/go,nawawi/go,CAFxX/go,nawawi/go,cloudflare/go,akutz/go,cloudflare/go,cloudflare/go,CAFxX/go,akutz/go,akutz/go,golang/go,nawawi/go,cloudflare/go,golang/go,cloudflare/go,golang/go,akutz/go