DOC: Clarify docstring for `sigma` parameter for `curve_fit`

Fixes #3682
Merge pull request #3920 from chatcannon/optimize_doc

DOC: optimize: Clarify docstring for `sigma` parameter for `curve_fit`
Fixed Docstring issues in linprog (Fixes #3905).

* removed superfluous second "Returns" section
* reordered the sections to match the docstring standard
Merge pull request #3922 from venthur/fix-3905

DOC: fix docstring issues in linprog (Fixes #3905).