encoding/json: clarify Marshal behavior for string keys of maps
encoding/json: clarify Marshal behavior for string keys of maps

This is a documentation-only change.

Fixes #28827

Change-Id: Ife9ab997809048784f35872b09905bc209a05eff
Reviewed-on: https://go-review.googlesource.com/c/go/+/188417
Reviewed-by: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>

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