On 'svn-auth-x509' branch, add comment to help avoid some ambiguity.

* subversion/libsvn_subr/x509parse.c
  (x509_get_version): Document that the version has the context specific tag
    of 0 so as to be clear that '| 0' is not a mistake but more a documentation.


git-svn-id: https://svn.apache.org/repos/asf/subversion/branches/svn-auth-x509@1635358 13f79535-47bb-0310-9956-ffa450edef68