spec: use Unicode terminology consistently
spec: use Unicode terminology consistently

- refer to character "categories" rather than "classes" per the
  definitions in the Unicode standard
- use "uppercase", "lowercase" (one word) instead of "upper case"
  or "upper-case", matching the spelling in the Unicode standard
- clarify that that the blank character "_" is considered a lowercase
  letter for Go's purposes (export of identifiers)

Fixes #44715.

Change-Id: I54ef177d26c6c56624662fcdd6d1da60b9bb8d02
Reviewed-on: https://go-review.googlesource.com/c/go/+/405758
Reviewed-by: Robert Griesemer <a6752097faebf1f02d80107d77a9be009089afe3@google.com>
Reviewed-by: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>
Reviewed-by: Robert Findley <9179eee875d1ee628c37fb47d7e749e855ffd21f@google.com>

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