Unify bandwidth related terms in dir-spec and Tor code.

Unifying these terms would make following what these bandwidth do in the code and the spec less confusing. For instance:

• what is called bandwidthrate [0] in the code when building the descriptor, it is called bandwidth-avg in dir-spec.txt [1].
• what is called bandwidthcapacity [2] in the code, it is called bandwidth-observed in dir-spec.txt[1]

I don't know if it is prefered to modify terms in the spec or in the code.

It'd be also helpful to add formulae or more detailed descriptions on how the different bandwidth terms are generated in dir-spec.txt

[0] ​https://gitweb.torproject.org/tor.git/tree/src/or/router.c#n2370 [1] ​https://gitweb.torproject.org/torspec.git/tree/dir-spec.txt#n427 [2] https://gitweb.torproject.org/tor.git/tree/src/or/router.c#n2376