Minor improvement to comments / docs for Symmetry/AllowedCouplings.jl

[ColeMiles]

The current comment at the top of Symmetry/AllowedCouplings.jl is a bit confusing -- it seems to imply that the constraints on allowed exchange matrices are:

$J = R J R^T$

for all R that map bonds within a symmetry equivalency class. But these constraints are only for R which map bonds to themselves.

Maybe as part of this issue, a new section in the "Internals" documentation could be made which explains the ideas of the symmetry analysis and how the algorithms work?

[kbarros]

Partially addressed in Commit ec0d584

Helped wanted to split this out into a nice Markdown document.

[kbarros]

I think the current code comments are good. Comprehensive markdown docs would of course be better, but that will require a separate effort.