#326 Update protocol documentation

[janwillemb]

The protocol documentation states the following:

"The framework should fail if the query has a function parameter with a different value than the one for the _function parameter"

The XINS framework does not implement this rule anymore; the function parameter is simply ignored. So, the documentation probably needs to be updated.

[janwillemb]

The rule applies when providing the function parameter twice with different values (although it's obsolete, as this parameetr is replaced by its _function counterpart).

[Anthony Goubard]

will be in xins 2.3-beta2