On the fsx branch:  Mark FSX as "experimental" in its docstring.
No functional change.

* subversion/include/svn_fs.h
  (SVN_FS_TYPE_FSX): expand docstring.

Suggested by: danielsh

git-svn-id: https://svn.apache.org/repos/asf/subversion/branches/fsx@1509593 13f79535-47bb-0310-9956-ffa450edef68