Fix broken sys.doc function

03d264b broke sys.doc by adding a '.' to the end of the module if it is
not an empty string. While this succeeds in making 'sys' not match the
sysctl modules functions, it also means that passing a specific function
name (ex. 'network.interfaces') would result in the passed value having
that same '.' appended (i.e. 'network.interfaces.', note the trailing
dot). This causes sys.doc to fail in returning the docstring for this
function since 'network.interfaces.' will not match any of the functions
in the __salt__ dict.
Merge pull request #3747 from archtaku/quickfix

Fix broken sys.doc function