clarify status of non-full-segment parameters

[rogpeppe]

The documentation does not make it clear whether
paths of the form:

/prefix:foo

are allowed. These are currently supported by httprouter,
but don't seem quite in the spirit of its simple specification
(you can't have statically defined suffixes for a path element,
so why should you be allowed a statically defined prefix?)

This form:

/prefix*foo

is similarly not specified but is explicitly disallowed. The two forms should
perhaps be made consistent with each other.

[mpnally]

Please don't fix this by disallowing /prefix:foo; I am using that capability