Followup to r31138:  Document what validations are done on properties.
Followup to r31138:  Document what validations are done on properties.

Suggested by: kfogel

* subversion/include/svn_repos.h
  (svn_repos_fs_change_node_prop):  Document property validations.
  (svn_repos_fs_change_rev_prop3, svn_repos_fs_change_txn_props):
    Point to svn_repos_fs_change_node_prop() for validation details.
  (svn_repos_fs_change_txn_prop):
    Point to svn_repos_fs_change_txn_props() for more information.

jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion