Fix docstrings that name parameters the operator methods do not have

[VenishPaneliya]

Four of the + operator methods document a parameter name their signature doesn't use, so following the docs and passing that keyword raises TypeError.

file	method	signature	documented
ggplot.py	ggplot.__add__	rhs	other
geoms/geom.py	geom.__radd__	other	plot
guides/guides.py	guides.__radd__	other	plot
stats/stat.py	stat.__radd__	other	gg

other is the established name here. Of the 24 __radd__ methods in the package, the two that document their parameter at all — ggplot.__radd__ and composition/_types.py — both use other; the remaining 19 have no Parameters section.

ggplot.py makes the case on its own: its __radd__ documents other correctly, while a few lines away __add__ also says other for a parameter that is actually called rhs.

I left stat.__radd__'s out : entry alone — that's the name under Returns, which is valid numpydoc, not a mistake.

Docs only, no behaviour change. ruff check and ruff format --check are clean on all four files.