doc/go1.19: move the description of the runtime.GOROOT change from 'cmd/go' to 'runtime'
doc/go1.19: move the description of the runtime.GOROOT change from 'cmd/go' to 'runtime'

Even though the change in the behavior of 'runtime.GOROOT' was
not actually due to a change in the runtime package proper, I
suspect that users who notice it will look for the release note
in that section, not the 'cmd/go' section.

Fixes #51461.

Change-Id: I271752968d4152a7fdf3e170537e3072bf87ce86
Reviewed-on: https://go-review.googlesource.com/c/go/+/400814
Reviewed-by: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@google.com>

nawawi/go,golang/go,golang/go,golang/go,golang/go,nawawi/go,nawawi/go,golang/go,nawawi/go,golang/go,nawawi/go,nawawi/go,golang/go,golang/go,nawawi/go,nawawi/go