Expand comment for HVecMultiplyAccumulate

State explicitly that fused multiply-add may not be used, since this
was a source of prior confusion. Add a DCHECK to draw developers
attention to this, if they try to add a floating point multiply-add.

See https://android-review.googlesource.com/c/platform/art/+/716505 .

Test: Treehugger
Change-Id: I1331be120a0a54baeb4da92e9211407b08892e98