Add cleaner explanation of environment mapping

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.
Merge pull request #7362 from piffio/gitfs-documentation-tips

Add cleaner explanation of environment mapping