This clarifies meaning of "case folding" Unicode equality with more familiar "case insensitive" wording.
For case folding properties see ftp://ftp.unicode.org/Public/UNIDATA/CaseFolding.txt.

Fixes #33447

Change-Id: I6ee85ab398679bf2a0b7d18693985ff0979d6c5a
GitHub-Last-Rev: accc9159330c61e046d77f77beac62b38bf72c19
GitHub-Pull-Request: golang/go#34434
Reviewed-on: https://go-review.googlesource.com/c/go/+/196717


Reviewed-by: Rob Pike <r@golang.org>