Add more explanation to parse_netloc()'s docstrings.

This addresses @pradyunsg's review comments.

src/pip/_internal/vcs/__init__.py

@@ -218,6 +218,11 @@ class VersionControl(object):
         Parse the repository URL's netloc, and return the new netloc to use
         along with auth information.
 
+        This is mainly for the Subversion class to override, so that auth
+        information can be provided via the --username and --password options
+        instead of through the URL.  For other subclasses like Git without
+        such an option, auth information must stay in the URL.
+
         Returns: (netloc, username, password).
         """
         return netloc, None, None

src/pip/_internal/vcs/subversion.py

@@ -133,6 +133,9 @@ class Subversion(VersionControl):
     def parse_netloc(self, netloc):
         """
         Parse out and remove from the netloc the auth information.
+
+        This allows the auth information to be provided via the --username
+        and --password options instead of via the URL.
         """
         if '@' not in netloc:
             return netloc, None, None