Mention that the Sphinx docs build wants version 1.5 or later

`misc.highlighting_failure` support was added to `suppress_warnings` in that
version, and the warnings-as-errors docs build relies on it.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@298277 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed