DOC: sparse.linalg: Fix lobpcg docstring.

Change the markup of the "Acknowledgements" title from a section heading
to emphasis markers within the "Notes" section.

The docstring standard doesn't include an "Acknowledgements" section,
so including one in a docstring generates a warning when building the
docs, and the section is not included in the HTML output.
Merge pull request #3892 from WarrenWeckesser/doc-lobpcg

DOC: sparse.linalg: Fix lobpcg docstring.
DOC: stats: Remove the trailing '_' from the reference to 'levene'.
DOC: stats: Use a more conventional description of the default value for 'moments' in the continuous distribution docstring.
DOC: stats: Fix a bad indent in the histogram docstring.
DOC: stats: Tweak the layout of the PDF function in the ncf docstring.
DOC: stats: Fix bad indent in the relfreq docstring.
Merge pull request #3894 from WarrenWeckesser/doc-stats

DOC: stats: Assorted docstring edits.