[POWERPC] Add documentation for the zImage's gunzip convenience functions

This patch adds documenting comments to the gunzip convenience
functions added in commit ad9d2716cf.
It also removes a stray newline, and an unused global variable.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Paul Mackerras <paulus@samba.org>

arch/powerpc/boot/gunzip_util.h

@@ -12,6 +12,22 @@
 
 #include "zlib.h"
 
+/*
+ * These functions are designed to make life easy for decompressing
+ * kernel images, initrd images or any other gzip compressed image,
+ * particularly if its useful to decompress part of the image (e.g. to
+ * examine headers) before decompressing the remainder.
+ *
+ * To use:
+ *     - declare a gunzip_state structure
+ *     - use gunzip_start() to initialize the state, associating it
+ *       with a stream of compressed data
+ *     - use gunzip_partial(), gunzip_exactly() and gunzip_discard()
+ *       in any combination to extract pieces of data from the stream
+ *     - Finally use gunzip_finish() to extract the tail of the
+ *       compressed stream and wind up zlib
+ */
+
 /* scratch space for gunzip; 46912 is from zlib_inflate_workspacesize() */
 #define GUNZIP_SCRATCH_SIZE	46912
 
@@ -27,4 +43,3 @@ void gunzip_discard(struct gunzip_state *state, int len);
 int gunzip_finish(struct gunzip_state *state, void *dst, int len);
 
 #endif /* _PPC_BOOT_GUNZIP_UTIL_H_ */
-