Clarify mp-value

[worley@…]

In section 6.3.5, the rules for the value part of the mp parameter of an hi-entry aren't given very clearly, but the idea seems to be that the value is the index of the hi-entry that contains the URI which was mapped into the request-URI of this hi-entry. There seem to be two cases:

• The mapping was done by the proxy on its own authority, in which case the mp-value is the parent index of the hi-entry's index.

• The mapping was done due to receiving a 3xx response, in which case the mp-value is an earlier sibling of the hi-entry's index, that of the downstream request which received the 3xx response.