Update docstring to reflect default setting in annotations

[teonbrooks]

Fixes #13295.

What does this implement/fix?

I modified the base docstring to signal the potential difference in default. It seems like all the other methods do default to None except the one for annotations. Instead of removing the default base docstring for all the other methods, I modified it to read unless otherwise specified and then added the specification to the annotation method.

Additional information

[drammock]

CI error suggests that the new bit is being treated as a separate param entry instead of part of the prior entry:

FAILED mne/tests/test_docstring_parameters.py::test_docstring_parameters - AssertionError: 2 errors found:
mne : mne.annotations.Annotations.to_data_frame : PR02 : Unknown parameters {'Default is ``datetime``.'}
mne : mne.annotations.Annotations.to_data_frame : PR07 : Parameter "Default is ``datetime``." has no description

debug locally with pytest mne/tests/test_docstring_parameters.py

doc build failure is same issue.

[drammock]

current doc build fix is unrelated, I'll push a commit shortly to deal with it

[drammock]

all green! thanks @teonbrooks