doc/go1.15: document two noteworthy json changes

doc/go1.15: document two noteworthy json changes

I had a look at the changes between 1.14 and master, and these are the
only two that seem relevant enough for the changelog.

There was also CL 179337 to reuse values when decoding map elements, but
it got reverted in CL 234559 and is not being included in 1.15.

Updates #37419.

Change-Id: Ib125415a953471ce29553a413d85aaf4b18a7a12
Reviewed-on: https://go-review.googlesource.com/c/go/+/236523
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>