doc: clean up Descriptors section

[tomlogic]

Addresses #58 and #59.

Need to do a few more things:

• Update to include new global _nibble property (Global nibble setting #60).

[tomlogic]

@francisdb, this is ready for review.

I've updated my Python implementation to support the new _nibble setting, and updated some of the files to use it instead of having a nibble property on every entry.

I think it might be possible to update normalize-map.py to identify maps that have an identical nibble setting on all entries, add a _nibble to the file, and then remove the nibble settings. I just manually edited the three map files of this PR.

[francisdb]

Looks good