Improve documentation of from_openmm and related features

[mattwthompson]

Description

In part resolves #1004 #1005 #1007

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 91.28%. Comparing base (6a1fb9e) to head (a3d93b4).
Report is 14 commits behind head on develop.

Additional details and impacted files

[mattwthompson]

Rendered docs: https://openff-docs--1008.org.readthedocs.build/projects/interchange/en/1008/using/edges.html

[mattwthompson]

Could I get a quick once-over (aka find my typos, even though I edited it once myself)?

[Yoshanuikabundi]

Two typos, and then a bunch of optional rephrasings to reduce parentheticals and keep a single logical flow. Looks good, this is very useful info! I've also suggested using a Sphinx cross-reference to link to the appropriate page, and I think the note might need to be moved to different functions?

[Yoshanuikabundi]

Alternate:

Suggested change

	The OpenFF Toolkit does not support isotopes or modifiying masses from the values defined in the periodic table. In the `Topology` and `Molecule` classes, particles masses are defined only by their atomic number. When topologies are read from OpenMM, the particle mass is ignored and the atomic number of the element is read and used to define the atomic properties.
	The OpenFF Toolkit does not support isotopes or modifying masses from the values defined in the periodic table. In the `Topology` and `Molecule` classes, particles masses are defined only by their atomic number. When topologies are read from OpenMM, the particle mass is ignored and the atomic number of the element is read and used to define the atomic properties.

[mattwthompson]

Closes #1006

[mattwthompson]

Note this is going into the 0.4 line, so these changes won't be reflected in 0.3 releases