document in the man-page that {ansible,ansible-playbook} -M can also be overriden with the ANSIBLE_LIBRARY environment

docs/man/man1/ansible-playbook.1.asciidoc.in

@@ -47,7 +47,8 @@ The 'PATH' to the inventory hosts file, which defaults to
 
 *-M* 'DIRECTORY', *--module-path=*'DIRECTORY'::
 
-The 'DIRECTORY' to load modules from. The default is '/usr/share/ansible'.
+The 'DIRECTORY' search path to load modules from. The default is
+'/usr/share/ansible'. See also the ANSIBLE_LIBRARY environment variable.
 
 *-e* 'VARS', *--extra-vars=*'VARS'::
 

docs/man/man1/ansible.1.asciidoc.in

@@ -61,7 +61,7 @@ Execute the module called 'NAME'.
 *-M* 'DIRECTORY', *--module-path=*'DIRECTORY'::
 
 The 'DIRECTORY' to load modules from. The default is '/usr/share/ansible'.
-
+See also the ANSIBLE_LIBRARY environment variable.
 
 *-a* \'_ARGUMENTS_', *--args=*\'_ARGUMENTS_'::