improve documents

Use consistent name. (Mingw -> MinGW)
fix indent
fix indent in examples
update document about VC
break long lines

tw is set to 78.

Improve explanation of condition in vimproc#pgroup_open().
fix style

A space is inserted before a '('.
{is-pty} argument was missing
move a tag before a function name

This is the same style as the official Vim help.
improve documents
Merge pull request #134 from k-takata/topic/improve-documents

improve documents