Version Control With Subversion Source

Finish issue #78 ("A fuller explanation of '--depth ARG' in section
'svn Options' would be useful").

* en/book/ch09-reference.xml
Provide quick descriptions of the depth values when describing the
--depth option. Add a pointer to the Sparse Directories section in
the description of --set-depth. Note the inconsistent depths at
which --non-recursive is applied to various subcommands.