powerpc: Update asm-prototypes.h comment

This header was recently cleaned up in commit 76222808 ("powerpc:
Move C prototypes out of asm-prototypes.h"), update the comment to
reflect it's proper purpose.
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20220617080243.2177583-1-mpe@ellerman.id.au

arch/powerpc/include/asm/asm-prototypes.h

@@ -2,8 +2,9 @@
 #ifndef _ASM_POWERPC_ASM_PROTOTYPES_H
 #define _ASM_POWERPC_ASM_PROTOTYPES_H
 /*
- * This file is for prototypes of C functions that are only called
- * from asm, and any associated variables.
+ * This file is for C prototypes of asm symbols that are EXPORTed.
+ * It allows the modversions logic to see their prototype and
+ * generate proper CRCs for them.
  *
  * Copyright 2016, Daniel Axtens, IBM Corporation.
  */