[DOC] Fix issues with DataFrame.aggregate page
Fixed some formatting issues. Description was gettign rendered in the
wrong location.

Moved a line from generic.py to frame.py to help with the rendering.
Moved the description being clubbed with see also to doc string with the
function declaration in frame.py

- closes #24668

-  passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
