doc: alphabetize cola.* variables

Signed-off-by: David Aguilar <davvid@gmail.com>
docs: document cola.terminal

git-cola uses `xterm -e` by default, but it can be changed
by setting the `cola.terminal` configuration variable.

Mention this in the documentation.

Closes #354

Reported-by: Dave Daynard <nardholio@gmail.com>
Signed-off-by: David Aguilar <davvid@gmail.com>