gallium: document conservative rasterization flags

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>

src/gallium/docs/source/screen.rst

@@ -456,10 +456,15 @@ The integer capabilities:
   as opposed to padding to vec4s.
 * ``PIPE_CAP_CONSERVATIVE_RASTER_POST_SNAP_TRIANGLES``: Whether the
   ``PIPE_CONSERVATIVE_RASTER_POST_SNAP`` mode is supported for triangles.
+  The post-snap mode means the conservative rasterization occurs after
+  the conversion from floating-point to fixed-point coordinates
+  on the subpixel grid.
 * ``PIPE_CAP_CONSERVATIVE_RASTER_POST_SNAP_POINTS_LINES``: Whether the
   ``PIPE_CONSERVATIVE_RASTER_POST_SNAP`` mode is supported for points and lines.
 * ``PIPE_CAP_CONSERVATIVE_RASTER_PRE_SNAP_TRIANGLES``: Whether the
   ``PIPE_CONSERVATIVE_RASTER_PRE_SNAP`` mode is supported for triangles.
+  The pre-snap mode means the conservative rasterization occurs before
+  the conversion from floating-point to fixed-point coordinates.
 * ``PIPE_CAP_CONSERVATIVE_RASTER_PRE_SNAP_POINTS_LINES``: Whether the
   ``PIPE_CONSERVATIVE_RASTER_PRE_SNAP`` mode is supported for points and lines.
 * ``PIPE_CAP_CONSERVATIVE_RASTER_POST_DEPTH_COVERAGE``: Whether

src/gallium/include/pipe/p_defines.h

@@ -644,7 +644,18 @@ enum pipe_reset_status
 enum pipe_conservative_raster_mode
 {
    PIPE_CONSERVATIVE_RASTER_OFF,
+
+   /**
+    * The post-snap mode means the conservative rasterization occurs after
+    * the conversion from floating-point to fixed-point coordinates
+    * on the subpixel grid.
+    */
    PIPE_CONSERVATIVE_RASTER_POST_SNAP,
+
+   /**
+    * The pre-snap mode means the conservative rasterization occurs before
+    * the conversion from floating-point to fixed-point coordinates.
+    */
    PIPE_CONSERVATIVE_RASTER_PRE_SNAP,
 };