This PR will bring the following changes to the weakref documentation:

• document the optional callback parameter of weakref.WeakMethod;
• normalise the use of ‘proxy’, replacing the mix of ‘proxy’ (8 occurrences) and ‘proxy object’ (3 occurrences);
• normalise the use of ‘weak reference’, replacing the mix of ‘weak reference’ (37 occurrences), ‘weak reference object’ (7 occurrences), ‘reference’ (7 occurrences), ‘reference object’ (6 occurrences), ‘weakref’ (3 occurrences), and ‘weakref object’ (1 occurrence).

https://bugs.python.org/issue47220