gallivm: add a few comments

src/gallium/auxiliary/gallivm/lp_bld_sample.h

@@ -56,14 +56,14 @@ struct lp_sampler_static_state
 {
    /* pipe_sampler_view's state */
    enum pipe_format format;
-   unsigned swizzle_r:3;
+   unsigned swizzle_r:3;     /**< PIPE_SWIZZLE_* */
    unsigned swizzle_g:3;
    unsigned swizzle_b:3;
    unsigned swizzle_a:3;
 
    /* pipe_texture's state */
-   unsigned target:3;
-   unsigned pot_width:1;
+   unsigned target:3;        /**< PIPE_TEXTURE_* */
+   unsigned pot_width:1;     /**< is the width a power of two? */
    unsigned pot_height:1;
    unsigned pot_depth:1;