Fix the release notes.

The Bug Fixes section is empty, which creates an empty Docbook section,
which isn't allowed. Comment out its header for now.

AsciiDoc interprets single quoted text as emphasized, which probably
isn't what was intended. Use curly quote markup instead.

Add a link to the AsciiDoc cheat sheet.

Change-Id: Ib9746ea714f90a28a2f1204e97e9e185aaef3df6
Reviewed-on: https://code.wireshark.org/review/4429
Reviewed-by: Gerald Combs <gerald@wireshark.org>