doc/go1.15: add notes for GOMODCACHE, modcacheunzipinplace
doc/go1.15: add notes for GOMODCACHE, modcacheunzipinplace

For #36568
For #34527

Change-Id: Ieea4b4a7644e9c957f48d08d2e172e39b571502f
Reviewed-on: https://go-review.googlesource.com/c/go/+/230537
Reviewed-by: Michael Matloob <4562438b12c14e81a47550acde616ff328a909f8@golang.org>
Reviewed-by: Bryan C. Mills <1c8aad60184261ccede67f5c63a0d2a3bf3c9ff4@google.com>

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