Add some docstrings

* darkroom.el:
(darkroom-increase-margins, darkroom-decrease-margins): Add docstring.
Prefix internal symbols with "darkroom--"

* darkroom.el (darkroom--compute-margins)
(darkroom--float-to-columns, darkroom-buffer-margins)
(darkroom--set-margins, darkroom--saved-mode-line-format)
(darkroom--saved-header-line-format, darkroom--saved-margins)
(darkroom-tentative-mode): Internal variables and functions now
prefixed "darkroom--".
Minor tweaks

* darkroom.el: Rewrite "Commentary" section
(darkroom-margins): Don't mention `darkroom--set-margins'.
(darkroom-guess-margins): Use `truncate', not `round'
Corrections after another review iteration with Rasmus

* darkroom.el (darkroom-margins): New default.
(darkroom-margins): Tweak docstring.
(darkroom-margins): Better type spec.
(darkroom-margins-if-failed-guess): New defcustom.
(darkroom--real-window-width): New function.
(darkroom-guess-margins): Simplified.
(darkroom--compute-margins)
(darkroom--saved-mode-line-format)
(darkroom--saved-header-line-format, darkroom--saved-margins): Add
docstring.
(darkroom-mode): Added an assertion.
(darkroom--tentative-mode-driving): New variable.
(darkroom--maybe-enable): Use `darkroom--tentative-mode-driving'
(darkroom-mode): Attempt to ensure `darkroom--set-margins' always
happens after `darkroom--maybe-enable' in
`window-configuration-change-hook'.