spec: consistently use "element" rather than "value" for map entry values
spec: consistently use "element" rather than "value" for map entry values

The spec refers to a map's key and element types; thus the respective
values are "keys" and "elements". Also, a map value is the value of
the entire map.

Similar fix for channels, where appropriate.

Fixes #23254.

Change-Id: I6f03ea6d86586c7b0b3e84f0c2e9446b8109fa53
Reviewed-on: https://go-review.googlesource.com/85999
Reviewed-by: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>
Reviewed-by: Matthew Dempsky <e56ac2c500064424afac424d4b5e53bd0570a731@google.com>
Reviewed-by: Russ Cox <5ad239cb8a44f659eaaee0aa1ea5b94947abe557@golang.org>
Reviewed-by: Rob Pike <4dc7c9ec434ed06502767136789763ec11d2c4b7@golang.org>

christopher-henderson/Go,nawawi/go,nawawi/go,Cofyc/go,akutz/go,CAFxX/go,golang/go,josharian/go,nawawi/go,Cofyc/go,Cofyc/go,nawawi/go,codestation/go,josharian/go,Cofyc/go,Cofyc/go,Cofyc/go,nawawi/go,codestation/go,CAFxX/go,josharian/go,CAFxX/go,cloudflare/go,josharian/go,akutz/go,akutz/go,cloudflare/go,nawawi/go,codestation/go,josharian/go,cloudflare/go,codestation/go,cloudflare/go,josharian/go,akutz/go,josharian/go,CAFxX/go,cloudflare/go,codestation/go,nawawi/go,CAFxX/go,golang/go,Cofyc/go,akutz/go,golang/go,CAFxX/go,cloudflare/go,josharian/go,nawawi/go,codestation/go,codestation/go,golang/go,codestation/go,Cofyc/go,CAFxX/go,akutz/go,golang/go,golang/go,golang/go,akutz/go,cloudflare/go,akutz/go,CAFxX/go,cloudflare/go,golang/go