magit-git-section: improve doc-string

Switch the doc-strings of `magit-git-section' and `magit-insert-section'
and make the necessary adjustment.  Previously doc-string of the former
revered to that of the latter but since Magit is about Git, not random
processes, it should be the other way around.
minor magit font-lock cleanup
rename magit-insert-{ => stgit-}series

Looking at the name should be enough to know this is from an extension.
rename magit-insert-{ => topgit-}topics

Looking at the name should be enough to know this is from an extension.
Also rename the name of the section from just `topics' to `stgit-topics'.
rename magit-{svn-insert => insert-svn}-{status- => }remote-line

For consistency with other such inserters in magit.el switch "svn" and
"insert".  Also remove the "status-" part.  While this is intended for
use in status buffer, that is not mean we need to keep that part.
Actually it should also be removed for some other inserters that contain
it, since they can be used outside the status buffer.
Merge branch 'section-cleanup'