Document new verbosity modifiers

While implementing #4518 I didn't find any mention of the new verbosity symbolic names and modifiers
as understood by `parseVerbosity` in the cabal manual (I would have expected them near e.g. http://cabal.readthedocs.io/en/latest/nix-local-build.html#cfg-flag---verbose)

For reference, these are currently

- `silent`
- `normal`
- `verbose`
- `debug`
- `deafening`

and

- `+callsite`
- `+callstack`
- `+nowrap`
- `+markoutput`
- `+timestamp`

Start with the Cabal manual's nix-local-build section near the --verbose flag documentation, then compare it with the verbosity names understood by parseVerbosity. Document the five symbolic names and five modifiers there; done when the manual explains every listed value and modifier.

Written by the indexing model from the issue text.