Improve docstrings in accordance with checkdoc

[bbatsov]

Many of the docstrings in flx.el and flx-ido.el at the moment do no comply to the established Emacs standards (M-x checkdoc) - it'd be great if they did.

I'd have updated them myself, but it's probably a better idea for @lewang to update them, since as the author he's capable to write the most precise descriptions.