etdoc extra-info stats items - tighten descriptions

extra-info-document-format.md defines many statistics reports from relays.

The stats report Items have a great deal of commonality. For example:

Interval syntax and semantics
Different Items with similar KEY=COUNT structure
Processed cell decile counts

I'm hoping that the apparent similarity corresponds to shared implementation in C Tor. In any case I'm hoping that the real protocol is consistent rather than diverging piecemeal. Regardless of whether that's the case, the spec structure ought to exploit the apparent similarity rather than recapitulating, and should therefore mention explicitly any divergences that currently exist.

To upload designs, you'll need to enable LFS and have an admin enable hashed storage. More information