util/format: document block depth field

After the pixel block width and height, a third field is used to
store the pixel block depth. Document this field.

Signed-off-by: Simon Ser <contact@emersion.fr>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Acked-by: Leo Liu <leo.liu@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/10134>

src/util/format/u_format.csv

@@ -34,6 +34,7 @@
 # - layout, per enum util_format_layout, in shortened lower caps
 # - pixel block's width
 # - pixel block's height
+# - pixel block's depth, in number of pixels
 # - channel encoding (only meaningful for plain layout), containing for each
 #   channel the following information:
 #   - type, one of