[doc] Clarify responsibility for fixing experimental target problems

Experimental targets are meant to be maintained by the community behind
the target. They are not monitored by the primary build bots. This
change clarifies that it is this communities responsibility for things
like test fixes related to the target caused by changes unrelated to
that target.

See http://lists.llvm.org/pipermail/llvm-dev/2020-February/139115.html
for a full discussion.

Reviewed by: rupprecht, lattner, MaskRay

Differential Revision: https://reviews.llvm.org/D74538