Fixed #12651: providing links to the  .expanding in .cum*
- [ x] closes #12561
- [ ] tests added / passed
- [ ] passes `git diff upstream/master | flake8 --diff`
- [x] whatsnew entry
#12561 : wrote the `.expanding` function int the `basics.rst`

xref #12673. Should close #12651 (not #12561).
 sectional for the expanding provided.
 move these commits to #12673 
