Let's add issue 4129 error wordings to relevant docs

Hello,

There is an entry #4129[1] in Subversion's issue tracker, an entry in
Subversion 1.8 release notes[2] and a short doc about the problem[3].
I don't see any wordings of the errors that could occur with this
problem in none of the mentioned texts. That is why it seems to me
that a user who encounters this issue does not have enough information
to be able to identify and resolve it.

The patch attached adds the wordings of the errors to SVN 1.8 release
notes[2] and issue4129 doc[3] that 'svnadmin verify' produces when it
detects the problem. I've already added a comment to [1].

I'm sure that these wordings will help users to find the resolution
via web search.

[1]: https://issues.apache.org/jira/browse/SVN-4129
[2]: http://subversion.apache.org/docs/release-notes/1.8.html#verify-issue4129
[3]: https://svn.apache.org/repos/asf/subversion/site/publish/docs/issue4129.txt

-- 
With best regards,
Pavel Lyalyakin
VisualSVN Team