document in the man-page that {ansible,ansible-playbook} -M can also be ...

...overriden with the ANSIBLE_LIBRARY environment.

This is a tiny branch that tweaks the man page to clarify that the -M parameter of ansible and ansible-playbook can also take a search path argument and that the ANSIBLE_LIBRARY environment can also be used to modify the module_path.

The rational is that I wrote a custom facts module and searched a bit before finding a way to set the -M parameter via the environment so that our custom facts module is always included (its detecting if the machine that it runs on is a VMWare virtual machine, if that is a generally useful feature I'm happy to contribute this small module as well in a seperate branch).