document in the man-page that {ansible,ansible-playbook} -M can also be overriden with the ANSIBLE_LIBRARY environment
use the same text for -M in ansible.1 and ansible-playbook.1
improve wording, thanks to Michael DeHaan
Merge pull request #3000 from mvo5/devel

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