document a few constants

[cosmicexplorer]

Broken out from #287.

1. Add documentation to several constants.
2. Put those constants in a limits submodule (this part can be reverted if desired).

[cosmicexplorer]

I'm aware that constants like these are supposed to be implementation details, but I like the idea of bringing these out into the open because some of them have implications on security (e.g. MAX_FORWARD_JUMPS can be mapped directly to a specific line in the public specification for the double ratchet encryption process). A future change might be to make these constants into default values and allow consumers to override those by providing an additional parameter to the methods in {group_cipher,session_cipher,session,sender_keys}.rs that currently consume these constants.

[cosmicexplorer]

So I spent some time to understand what each of these limits actually represents, and I think the more involved explanations I have now are useful, in particular the description of what each limit actually bounds and why that matters (avoiding denial-of-service attacks, for one). I'm going to spend some more time understanding what an archived session state is.

While these descriptions are quite useful for me to understand what the library is doing (as implementation notes for some parts which aren't fully defined in the Double Ratchet spec), they may be too verbose for this file, so one possibility might be to move this more involved documentation to {session,group}_cipher.rs and just leave the pared-down explanation of what each limit actually bounds in these module docs.

If that arrangement seems useful, then we might also consider making limits a pub mod in order to expose these docs? Not sure right now. Going to figure out how session archiving works first before thinking about where to put these docs.

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