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>

Documentation/filesystems/path-lookup.txt

@@ -379,4 +379,4 @@ Papers and other documentation on dcache locking
 
 2. http://lse.sourceforge.net/locking/dcache/dcache.html
 
-
+3. path-lookup.md in this directory.