Tweak slime.el comments to be more MELPA-friendly

@ellerh This minor commit tweaks the comments a little bit to follow the usual header format for the description field (aka commentary) of elisp files. The motivation is that MELPA understands that ad-hoc format and propagates the description to here: http://melpa.org/#/slime. I tried to make this change play well with the outline setup and took the opportunity to update the text based on the SLIME manual's introduction. Does this look OK?