Update comment for ARM64.

upb/decode_fast.c

@@ -1,8 +1,8 @@
-// Fast decoder: ~3x the speed of decode.c, but x86-64 specific.
+// Fast decoder: ~3x the speed of decode.c, but requires x86-64/ARM64.
 // Also the table size grows by 2x.
 //
-// Could potentially be ported to ARM64 or other 64-bit archs that pass at
+// Could potentially be ported to other 64-bit archs that pass at least six
-// least six arguments in registers.
+// arguments in registers and have 8 unused high bits in pointers.
 //
 // The overall design is to create specialized functions for every possible
 // field type (eg. oneof boolean field with a 1 byte tag) and then dispatch