Improve REL_SOURCE description

[edublancas]

Hi, thanks for doing this, it's really useful!

Just a suggestion:

I was trying to generate the docs using:

sphinx-versioning build docs docs/build/html

But I was getting...

=> No docs found in any remote branch/tag. Nothing to do.

I couldn't fix it until I read the source code and realized that REL_SOURCE looks for the sphinx conf.py file to determine if the tag/branch has docs or not, but the description for REL_SOURCE in the build command says this:

 REL_SOURCE is the path to the docs directory relative to the git root. If
  the source directory has moved around between git tags you can specify
  additional directories.

I would be better to explain that the docs are found by using the conf.py cause it may be in other places, for example I have mine in docs/source/