user-manual: reorganize public git repo discussion

user-manual: reorganize public git repo discussion

Helping a couple people set up public repos recently, I wanted to point
them at this piece of the user manual, but found it wasn't as helpful as
it could be:

	- It starts with a big explanation of why you'd want a public
	  repository, not necessary in their case since they already knew
	  why they wanted that.  So, separate that out.
	- It skimps on some of the git-daemon details, and puts the http
	  export information first.  Fix that.

Also group all the public repo subsections into a single section, and do
some miscellaneous related editing.

Signed-off-by: "J. Bruce Fields" <bfields@citi.umich.edu>