Use consistent math notation in docs

[jeremiahpslewis]

Open questions:

• Inverse via Unicode or Latex
• Syntax for matrix transpose

[gdalle]

More generally, I think the docs contain a (Gaussian) mixture of LaTeX and Unicode symbols, so a global convention would be lovely

[cscherrer]

Sections not accessible from REPL "help mode" should be LaTeX I think, no reason not to do that. But I don't know of a way for method docstrings to look reasonable in a REPL with LaTeX. So maybe unicode for those? What do you think @mschauer ?