[PATCH] svn_fs.h doc updates on 'nodes' and 'node revisions'

Please could someone sanity-check this doc clarification. I've been a
bit unsure about the exact relationship between node-revs and
node-rev-ids and nodes and node-ids and the rest of the FS, and if I've
understood right this should help.

(I need to re-write the "... node and node-rev are used interchangeably"
sentence. I want to explain why that it's OK in certain places for a doc
string to talk about "a node" where the corresponding API parameter uses
a node-rev-id.)

- Julian