\subsetneq and \supsetneq not in docs

[Stevenparsons]

Reported by nageh:

Figured that \subsetneq and \supsetneq are supported as part of the
AMS package though it's not listed among the supported LaTeX commands.

We should probably run another sanity check to see if anything else is left out.

[Stevenparsons]

I verified they are missing on mathjax.org and the rst sources.

[dpvc]

These have been added in the latest version of the documentation, so I'm closing the issue.