Further suggestions for fn:path

[michaelhkay]

Several good suggestions for fn:path made at today's review:

(a) For namespaces, an option to identify elements by the result of the name() function - that is, using the actual prefix of each element, rather than a prefix obtained from an externally supplied map
(b) The ability to get a path to a node from a supplied ancestor rather than from the root. (Defaulting perhaps to a path from the context node, though that interacts awkwardly with the default for the target node itself.)
(c) Some more advice and guidance, especially relating to the different use cases depending on whether the path is for use by software or for (diagnostic?) use by human developers.