x86: zboot: Tidy up the comment for zboot_run()

The current use case (ChromeOS) is the uncommon case. Document how this
function is more normally used, where base is 0 and cmdline is NULL

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/include/bootm.h b/include/bootm.h
index a9069d7..d224e4a 100644
--- a/include/bootm.h
+++ b/include/bootm.h
@@ -143,11 +143,14 @@
  *
  * @addr: Address where the bzImage is moved before booting, either
  *	BZIMAGE_LOAD_ADDR or ZIMAGE_LOAD_ADDR
- * @base: Pointer to the boot parameters, typically at address
- *	DEFAULT_SETUP_BASE
+ * @size: Size of bzImage, or 0 to detect this
  * @initrd: Address of the initial ramdisk, or 0 if none
  * @initrd_size: Size of the initial ramdisk, or 0 if none
- * @cmdline: Command line to use for booting
+ * @base_addr: If non-zero, this indicates that the boot parameters have already
+ *	been loaded by the caller to this address, so the load_zimage() call
+ *	in zboot_load() will be skipped when booting
+ * @cmdline: If non-NULL, the environment variable containing the command line
+ *	to use for booting
  * Return: -EFAULT on error (normally it does not return)
  */
 int zboot_run(ulong addr, ulong size, ulong initrd, ulong initrd_size,