[3.12] Give `poplib.POP3.rpop` a proper docstring (GH-127370) (#127722)

Give `poplib.POP3.rpop` a proper docstring (GH-127370)

Previously `poplib.POP3.rpop` had a "Not sure what this does" docstring, now it has been fixed.
(cherry picked from commit 27d0d2141319d82709eb09ba20065df3e1714fab)

Co-authored-by: Stan Ulbrych <89152624+StanFromIreland@users.noreply.github.com>