aarch64: Add an extra comment to arm_sve.h

I've had a couple of conversations now in which the shortness
of arm_sve.h was causing confusion, with people thinking that
the types and intrinsics were missing.  It seems worth adding
a comment to explain what's going on.

2020-04-24  Richard Sandiford  <richard.sandiford@arm.com>

gcc/
	* config/aarch64/arm_sve.h: Add a comment.