Version Control With Subversion Source

* en/book/ch09-reference.xml
Document the svnadmin --memory-cache-size option.

Finish issue #47 ("1.7: document SVNAdvertiseV2Protocol mod_dav_svn
directive"), and then some.

* en/book/ch09-reference.xml
Document mod_dav_svn's new directives:
SVNAdvertiseV2Protocol
SVNCacheFullTexts
SVNCacheTextDeltas
SVNCompressionLevel
SVNInMemoryCacheSize

* en/book/ch06-server-configuration.xml
(svn.serverconfig.httpd.extra.writethruproxy.caveats): Note the
general problem that could occur with version mismatches in
write-through proxy situations, and mentioned
SVNAdvertiseV2Protocol. (NOTE: I think there is still another
problem with revprop atomicity that is not so easily worked
around.)

* en/book/ch09-reference.xml
Merge from /trunk/en r4024, which alphabetized the mod_dav_svn and
mod_authz_svn directives.

* en/book/ch09-reference.xml
Alphabetize mod_dav_svn and mod_authz_svn directives. No content changes.

Add a missing closing tag.

* en/book/ch09-reference.xml
Reference docs for 'svn merge --allow-mixed-revisions'.

* en/book/ch09-reference.xml
Doc 'svn export --ignore-keywords'.

* en/book/ch09-reference.xml
Document 'svnversion --quiet' (which is apparently only useful when
combined with --version). Ho hum.

* en/book/ch09-reference.xml
Document svnsync's --config-option, --disable-locking,
--source-prop-encoding, and --steal-locks options.

* en/book/ch09-reference.xml
Clarify in the warnings about not using explicit source URLs with
'svnsync sync' and 'svnsync copy-revprops' that the danger is with
untrusted users who can change the special r0 props, not just any
ol' committer.

Suggested by: danielsh