Docstring and identation cleanup

I've corrected a few docstrings, added a few more and fixed all indentation errors I could spot. I think I've made some decent progress, but we still have a long way to go until the entire codebase is aligned with the [official documentation guidelines](http://www.gnu.org/software/emacs/manual/html_node/elisp/Documentation-Tips.html#Documentation-Tips). Hopefully one day the output of running `checkdoc` in `nrepl.el` will not produce any warnings (currently there are about 140 of those). :-)