DOC: stats: fix spelling of 'Saunders' in 'Birnbaum-Saunders', and add reference.
DOC: stats: fix typo in the module docstring.
DOC: stats: fix isf docstring: isf is not 1-sf.
DOC: stats: Change layout of the beta PDF formula.
DOC: stats: fix typo in powerlaw distribution docstring.
Merge pull request #3953 from WarrenWeckesser/doc-stats-corrections

DOC: stats: Several corrections and small additions to docstrings.