Some documentation fixes and changes

1. Fixed some lines which were wider than 78 characters.
2. Added a couple of more borders above all the sub-sections.
3. Moved about part above the TOC, seems unnecessary to have that as its
own section when it's so small. This also look more like a  default vim
help file.
4. Changed "NOTE:" into "Note that" because that is more like how it's
used in default vim files.
5. Changed some wording like "the plug-in manager" to "a
plug-in manager".
6. Moved a couple of *tag names* below their headings so it's like that
everywhere in the file.
Language fixes after notes from @starcraftman
Merge pull request #385 from marlun/doc-fixes

Some documentation fixes and changes to the VimDocs.