This adds a brief explanation of git_pillar to the gitfs walkthrough.
Even though they don't work the same, they do similar things for
file_roots and pillar_roots, and it bears mentioning.
Also, this adds a better explanation of how git_pillar works to the
top-level docstring for pillar/git_pillar.py.
Explicitely state that the 'master' branch is automagically mapped
to the 'base' environment.
Also mention the fact that top.sls files are merged from the different
branches, which suggests a setup where top.sls is only present in
the 'master' branch.
This completes the documentation audit begun in cf365f7, using proper
backticks and :strong: tags to emphasize text, converting groups of
shell commands to bash code-block sections, etc. This also replaces the
use of \* with the easier-to-grok '*', which will hopefully reduce
confusion among new users.
