DOC: typos in BPoly docstring

[ci skip]
DOC: add a note about the sign of the shape parameter to genextreme.

closes gh-3844, closes gh-1851

[ci skip]
Merge pull request #3879 from ev-br/interp_docs

Small tweaks to the docs:

  -  fix a typo in BPoly docstrings
  -  add to the docstring of genextreme a cautionary note about the sign of the shape parameter.