Documentation: add new description of path-name lookup.

This document is based on three recent lwn.net articles.
Some of the introductory material and linkage between articles
has been removed, and some time-based descriptions have been
revised.

Also all links to code have been removed as the code is very close by.

Contains corrections and improvements from Randy Dunlap <rdunlap@infradead.org>.

Signed-off-by: NeilBrown <neil@brown.name>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>