doc: include fix for #34437 in Go 1.14 release notes [1.15 backport]

[dmitshur]

@dmitshur requested issue #38801 to be considered for backport to the next 1.15 minor release.

This is a release note backport for 1.14, and it needs to be backported to 1.15 too now.

[gopherbot]

Change https://golang.org/cl/252618 mentions this issue: [release-branch.go1.15] doc/go1.14: document json.Umarshal map key support of TextUnmarshaler

[cagedmantis]

Approved as this is a documentation fix.

[gopherbot]

Closed by merging 60fdbce to release-branch.go1.15.