Update to docs for **

Hi,
here's the slight docs modification for **.

I badly needed this feature for [this](https://github.com/capistrano/sshkit) project. I combed the `vim-projectionist` docs, but I still wasn't sure how `"test/**/test_*.rb"` works.
Due to wording of current docs: `...the inner portion is collapsed to a single slash...` I though that with `test/foo/test_baz.rb` I would get `"/baz".

I did the homework and read some previous issues before submitting a new one. [This](https://github.com/tpope/vim-projectionist/issues/14#issuecomment-41750115) comment nailed it.

Also, would you be open to expanding this change to include an example? `**` is kind of an advanced feature so it might deserve it.