font/sfnt: add more package docs
font/sfnt: add more package docs

Change-Id: Idebffba4a9260888914cff97d9b56c7ea392b139
Reviewed-on: https://go-review.googlesource.com/c/image/+/292249
Trust: Nigel Tao <58ae08b3356928ff9c67562b4719487d54fe8348@golang.org>
Reviewed-by: Dmitri Shuralyov <d181b7fea0ec87c86ba5a890ab716db52498e3ba@golang.org>

golang/image