Transition machinery for magit-insert-pending-changes

If users have customized their `magit-status-sections-hook` then it can
be confusing when they update magit and suddenly start getting error
messages due to `magit-insert-pending-changes` not being defined
anymore. Define a an obsolete function alias to a shim function that
displays a message to the user about what's going on.

See also: 382351845eca2425713f640f9bb55756c9ddf723