Added FILE_COMMON_DOCSTRINGS for common options.

Multiple modules share the same common arguments that are expressed
in code by the add_file_common_args argument in the AnsibleModule
class. The modules that set this argument to True don't always update the
documentation accordingly though. For example, the owner is sometimes
assumed to exist only from the examples. This branch proposes a way to
share documentation across modules by using a FILE_COMMON_DOCSTRINGS
instead of copying and pasting the same documentation across modules.