doc/go1.17: document fixes for negative rune handling

CL 317273 accidentally grouped a fix for bufio, bytes, strings
packages into a single entry, but they should be separate ones.

Fix that, and document these negative rune handling fixes.

The list of fixed functions in package unicode was computed by
taking the functions covered by the new TestNegativeRunes test,
and including those that fail when tested with Go 1.16.3.

For #44513.
Updates #43254.

Change-Id: I6f387327f83ae52543526dbdcdd0bb5775c678bd
Reviewed-on: https://go-review.googlesource.com/c/go/+/317469


Reviewed-by: David Benjamin <davidben@google.com>
Reviewed-by: Alexander Rakoczy <alex@golang.org>
Trust: Alexander Rakoczy <alex@golang.org>
Trust: Dmitri Shuralyov <dmitshur@golang.org>
Run-TryBot: Alexander Rakoczy <alex@golang.org>