Version Control With Subversion Source

In the book, cross-reference to "svn mkdir" from a point where it is likely to
be useful.
Based on a patch from John R. Daily <john_at_geekhavoc.com>.

* doc/book/book/ch04.xml (svn-ch-4-sect-6.1): Add xref to "svn mkdir".
* doc/book/book/ch09.xml (all command refentry tags): Add id attributes.
Remove some incorrect whitespace.