[DOC] Backtick and directive consistency.
[DOC] Backtick and directive consistency.

Some of the math in those docstrings was not using consistent directive;
it was sometime using double-backticks (verbatim), sometime using
simple-backticks (default directive, reference), and sometime
:math:-backticks which I believe is the correct one.

Where I spoted issues I now use :math:... consistently.

e-q/scipy,vigna/scipy,mdhaber/scipy,mdhaber/scipy,andyfaff/scipy,matthew-brett/scipy,grlee77/scipy,endolith/scipy,tylerjereddy/scipy,endolith/scipy,rgommers/scipy,perimosocordiae/scipy,WarrenWeckesser/scipy,scipy/scipy,andyfaff/scipy,perimosocordiae/scipy,e-q/scipy,perimosocordiae/scipy,e-q/scipy,Eric89GXL/scipy,Eric89GXL/scipy,endolith/scipy,rgommers/scipy,perimosocordiae/scipy,ilayn/scipy,WarrenWeckesser/scipy,Eric89GXL/scipy,Stefan-Endres/scipy,mdhaber/scipy,vigna/scipy,zerothi/scipy,WarrenWeckesser/scipy,Eric89GXL/scipy,anntzer/scipy,matthew-brett/scipy,scipy/scipy,perimosocordiae/scipy,anntzer/scipy,tylerjereddy/scipy,scipy/scipy,matthew-brett/scipy,tylerjereddy/scipy,perimosocordiae/scipy,andyfaff/scipy,grlee77/scipy,Stefan-Endres/scipy,grlee77/scipy,mdhaber/scipy,rgommers/scipy,Stefan-Endres/scipy,rgommers/scipy,andyfaff/scipy,anntzer/scipy,vigna/scipy,anntzer/scipy,ilayn/scipy,andyfaff/scipy,endolith/scipy,Eric89GXL/scipy,e-q/scipy,ilayn/scipy,rgommers/scipy,Eric89GXL/scipy,mdhaber/scipy,zerothi/scipy,zerothi/scipy,matthew-brett/scipy,anntzer/scipy,zerothi/scipy,matthew-brett/scipy,Stefan-Endres/scipy,tylerjereddy/scipy,ilayn/scipy,anntzer/scipy,endolith/scipy,scipy/scipy,zerothi/scipy,zerothi/scipy,WarrenWeckesser/scipy,grlee77/scipy,scipy/scipy,tylerjereddy/scipy,e-q/scipy,scipy/scipy,andyfaff/scipy,Stefan-Endres/scipy,vigna/scipy,WarrenWeckesser/scipy,ilayn/scipy,vigna/scipy,Stefan-Endres/scipy,endolith/scipy,grlee77/scipy,mdhaber/scipy,WarrenWeckesser/scipy,ilayn/scipy