Version Control With Subversion Source

Add new 'swig-py-ext-clean' rule, and attach to the higher-level
clean rules.

* doc/book/book/ch05.xml
* doc/book/book/ch07.xml
Include a couple of fixes suggested by Erik Sjölund <erik.sjolund@...>.

* subversion/svnlook/main.c
(cmd_table): Clear up some usage messages for svnlook subcommands.
Thanks (again) to Erik for pointing out the ambiguity of
'dirs-changed' vs. the directories printed by 'changed'