largefiles: improve comments, internal docstrings
- fix some ungrammatical/unclear/incorrect comments/docstrings
- rewrite some really unclear comments/docstrings
- make formatting/style more consistent with the rest of Mercurial
(lowercase without period unless it's really multiple sentences)
- wrap to 75 columns
- always say "largefile(s)", not "lfile(s)" (or "big files")
- one space between sentences, not two