Describe query recovery

[Aaron1011]

The section on incremental compilation mentions 're-running' a query when one of its inputs is red. We should describe this process ("recovery") in detail. In particular, we should note that this is currently only possible when the query ID can be mapped to a DefPathHash (this is the case for CrateNum, LocalDefId, and DefId).

[spastorino]

@Aaron1011 agreed, it would be great to document that in more detail.