Tweak release docs

Contrast releases to git snapshot tarballs.  Mention that
releases have some quality-improvement focus, with special
non-"dev" version IDs.  Explain more about version IDs,
using "openocd -v" to see them, etc;

Make release milestone info be less specific about timing,
and presume we have both a merge window and an RC stage.

Rework the release process information to match reality a
bit more closely.  Reference the version.sh script (in one
place the wrong script was referenced).  Bugfix branches
get special treatment, while non-bugfix releases are more
or less what *defines* being the mainline branch.

Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>