docs/conf.py: Suppress sphinx highlighting failure warnings

The pygments syntax highlighting package used by sphinx fails to parse
newer LLVM constructs or valid (at least to me) gas constructs like
`.secrel32 _function_name + 0`.

Disable this particular warning so the build doesn't abort as fixing
pygments doesn't seem a workable option here.

Differential Revision: https://reviews.llvm.org/D29794

llvm-svn: 294672