remove interactive spec, docstring cleanup
check mode var at the right time, simplify timers

The mode var check fixes an issue w/ automatically changing the major
mode in a post-command-hook.  If this happened after indent-guide-mode's
hook function (like for any global hook value, eg), the timer might get
started but would run in a major mode where indent-guide should be
disabled.

With this change, we can use a simpler repeating idle timer.  It only
needs to be removed when the mode is changed, since leftover timers can
no longer do any harm.