Improve documentation of Path

[uckelman]

Motivation

This improves the documentation for Path by specifically calling out that one Path extractor captures all components for a route. New users fall into the trap of thinking that each capture needs its own Path argument., which does not work.

See #2313 for more details.

Solution

This PR adds two sentences to the docs to clarify that only one Path should be used.

Fixes #2313

[davidpdrsn]

Thanks!