The comments on the category range tables in the unicode package are fairly
redundent and require an external source to translate into human readable
category names.

This adds a look up table with the category descriptions and uses it if
available when generating the comments for the range tables.

Fixes #28954

Change-Id: I853e2d270def6492c2c1dd2ad0ec761a74c04e5d
Reviewed-on: https://go-review.googlesource.com/c/151297Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>