html/template, text/template: document glob semantics

This change updates the doc comments of the various ParseGlob functions
and methods to state that they use the semantics of filepath.Match when
matching the file name pattern.

Fixes #30608

Change-Id: Iee4bdc0a2a2f8647d1f9a910e4d72a5de9204d11
Reviewed-on: https://go-review.googlesource.com/c/go/+/179739


Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>