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.