Clarify the use of hyphens in glyph names

[iterumllc]

Description

Having hyphens at the end of "development" glyph names makes handling ranges correctly in the grammar significantly trickier; the current implementation doesn't attempt to. As there doesn't seem to be a need for them in that location this PR updates the docs to clarify that they can't end a name.

Perhaps more controversially it also states that the special characters can't be used at the start of a name. The spec was ambiguous about this but the featgram.g implementation doesn't allow them at the beginning of a name. I agree with that decision—too dicey to allow them there.

One additional question is whether we should go farther and restrict the use of more or all of the special characters at the end of a name. The main reason for doing that would be to reserve potential glyph operators for future use, which could be handled the way hyphens are handled now. I leave that up for discussion.

Checklist:

• [x] I have followed the Contribution Guidelines
• [ ] I have added test code and data to prove that my code functions correctly
• [ ] I have verified that new and existing tests pass locally with my changes
• [ ] I have performed a self-review of my own code
• [ ] I have commented my code, particularly in hard-to-understand areas
• [x] I have made corresponding changes to the documentation

Closes #970

[skef]

I'll update this and assign @kaydeearts soon