sandbox: document parameters of os_realloc()

Avoid 'make htmldocs' build warnings:

    ./include/os.h:139: warning:
    Function parameter or member 'ptr' not described in 'os_realloc'
    ./include/os.h:139: warning:
    Function parameter or member 'length' not described in 'os_realloc'

Fixes: 14e46dfb17 ("sandbox: Add os_realloc()")
Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Reviewed-by: Simon Glass <sjg@chromium.org>

include/os.h

@@ -134,7 +134,9 @@ void os_free(void *ptr);
  * This follows the semantics of realloc(), so can perform an os_malloc() or
  * os_free() depending on @ptr and @length.
  *
- * Return:	Pointer to reallocated memory or NULL if @length is 0
+ * @ptr:	pointer to previously allocated memory of NULL
+ * @length:	number of bytes to allocate
+ * Return:	pointer to reallocated memory or NULL if @length is 0
  */
 void *os_realloc(void *ptr, size_t length);