pixel-formats: Document pixel format human-friendly conversion methods

Signed-off-by: Marius Vlad <marius.vlad@collabora.com>

libweston/pixel-formats.c

@@ -387,6 +387,10 @@ pixel_format_get_info_shm(uint32_t format)
 		return pixel_format_get_info(format);
 }
 
+/** Retrive a pixel format information structure from a DRM FOURCC format
+ *
+ * \param format a DRM FOURCC format
+ */
 WL_EXPORT const struct pixel_format_info *
 pixel_format_get_info(uint32_t format)
 {
@@ -427,6 +431,15 @@ pixel_format_is_opaque(const struct pixel_format_info *info)
 	return !info->opaque_substitute;
 }
 
+/** Retrieve the opaque substitute for a pixel format
+ *
+ * If the given pixel format contains an alpha channel, look up an identical
+ * pixel format except where the alpha channel is ignored, if such format
+ * exists. Otherwise returns the passed in format as is.
+ *
+ * \param info a pixel_format_info already retrieved using pixel_format_get_info()
+ *
+ */
 WL_EXPORT const struct pixel_format_info *
 pixel_format_get_opaque_substitute(const struct pixel_format_info *info)
 {