htpasswd docs: add notes about depending on python-passlib

library/web_infrastructure/htpasswd

@@ -54,6 +54,10 @@ options:
       - Used with C(state=present). If specified, the file will be created
         if it does not already exist. If set to "no", will fail if the
         file does not exist
+notes:
+  - This module depends on the I(passlib) Python library, which needs to be installed on all target systems.
+  - On Debian, Ubuntu, or Fedora: install I(python-passlib).
+  - On RHEL or CentOS: Enable EPEL, then install I(python-passlib).
 requires: [ passlib>=1.6 ]
 author: Lorin Hochstein
 """