Small get_tib_address cleanups

I noticed a non-bool-like use of target_get_tib_address in
windows-tdep.c.  After fixing this I thought it would be good to
document the target method; and this also lead to some non-bool-like
commentary in remote.c.  This patch fixes all of these nits.

Approved-By: Simon Marchi <simon.marchi@efficios.com>