Updated MyOpenMath documentation in Guide and sample-article

[mafitch]

Added details about MOM problems that may be problematic. Added use of parameters in sample-article.

[rbeezer]

Thanks, @mafitch, that'll be a big help. And it is always desirable (required?) to have tests in the sample article of new features (@params). (I didn't know that had been added!)

• Repackaged as two commits, as I do.
• Tried regenerating the changed problem for the static versions in the repository (even if I expected no real change). I don't use the CLI in development, so don't regenenerate automatically. See pretext-dev.
• Maybe you want your commits attributed with a more long-term email address, not Mark Fitch <58183892+mafitch@users.noreply.github.com>?
• Will update Guide online soon, but not immediately.

Thanks again, looking forward to more!