Reword comment in lz4_compress_zfs

The comment in lz4_compress_zfs could be more clear and specific.  It
also contains needlessly strong language.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Serapheim Dimitropoulos <serapheim@delphix.com>
Signed-off-by: Matthew Ahrens <mahrens@delphix.com>
Closes: openzfs#8702
Closes: openzfs#8703

module/zfs/lz4.c

@@ -63,10 +63,10 @@ lz4_compress_zfs(void *s_start, void *d_start, size_t s_len,
 		return (s_len);
 
 	/*
-	 * Encode the compressed buffer size at the start. We'll need this in
-	 * decompression to counter the effects of padding which might be
-	 * added to the compressed buffer and which, if unhandled, would
-	 * confuse the hell out of our decompression function.
+	 * The exact compressed size is needed by the decompression routine,
+	 * so it is stored at the start of the buffer. Note that this may be
+	 * less than the compressed block size, which is rounded up to a
+	 * multiple of 1<<ashift.
 	 */
 	*(uint32_t *)dest = BE_32(bufsiz);