Mention of ansible-doc and man pages for modules.
Merge pull request #2447 from dorfsmay/20130318-ansible-doc

Mention of ansible-doc and man pages for modules.
invalidate group cache so that a group added by the add_host module will be relfected in the groups variable in later plays.
Merge pull request #2457 from robparrott/add_host-inventory-group-fix

invalidate group cache when adding a new group