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.

gcc/ChangeLog

+2020-04-24  Richard Sandiford  <richard.sandiford@arm.com>
+
+	* config/aarch64/arm_sve.h: Add a comment.
+
 2020-04-24  Haijian Zhang <z.zhanghaijian@huawei.com>
 
 	PR rtl-optimization/94708

gcc/config/aarch64/arm_sve.h

@@ -32,6 +32,11 @@ typedef __fp16 float16_t;
 typedef float float32_t;
 typedef double float64_t;
 
+/* NOTE: This implementation of arm_sve.h is intentionally short.  It does
+   not define the SVE types and intrinsic functions directly in C and C++
+   code, but instead uses the following pragma to tell GCC to insert the
+   necessary type and function definitions itself.  The net effect is the
+   same, and the file is a complete implementation of arm_sve.h.  */
 #pragma GCC aarch64 "arm_sve.h"
 
 #endif