video: add video blend helper functions to docs

I don't think those were ever meant to be made public,
but they are, so we might as well document them.

docs/libs/gst-plugins-base-libs-sections.txt

@@ -2408,6 +2408,11 @@ GST_VIDEO_TILE_MAKE_STRIDE
 GST_VIDEO_TILE_X_TILES
 GST_VIDEO_TILE_Y_TILES
 
+#video-blend.h
+<SUBSECTION>
+gst_video_blend
+gst_video_blend_scale_linear_RGBA
+
 #video-converter.h
 <SUBSECTION>
 gst_video_converter_new

gst-libs/gst/video/video-blend.c

@@ -146,6 +146,22 @@ G_STMT_START { \
   ret = v0 + (v1 * (255 - alpha)) / 255; \
 } G_STMT_END
 
+/**
+ * gst_video_blend_scale_linear_RGBA:
+ * @src: the #GstVideoInfo describing the video data in @src_buffer
+ * @src_buffer: the source buffer containing video pixels to scale
+ * @dest_height: the height in pixels to scale the video data in @src_buffer to
+ * @dest_width: the width in pixels to scale the video data in @src_buffer to
+ * @dest: (out): pointer to a #GstVideoInfo structure that will be filled in
+ *     with the details for @dest_buffer
+ * @dest_buffer: (out): a pointer to a #GstBuffer variable, which will be
+ *     set to a newly-allocated buffer containing the scaled pixels.
+ *
+ * Scales a buffer containing RGBA (or AYUV) video. This is an internal
+ * helper function which is used to scale subtitle overlays, and may be
+ * deprecated in the near future. Use #GstVideoScaler to scale video buffers
+ * instead.
+ */
 /* returns newly-allocated buffer, which caller must unref */
 void
 gst_video_blend_scale_linear_RGBA (GstVideoInfo * src, GstBuffer * src_buffer,
@@ -232,7 +248,8 @@ gst_video_blend_scale_linear_RGBA (GstVideoInfo * src, GstBuffer * src_buffer,
   g_free (tmpbuf);
 }
 
-/* video_blend:
+/**
+ * gst_video_blend:
  * @dest: The #GstVideoFrame where to blend @src in
  * @src: the #GstVideoFrame that we want to blend into
  * @x: The x offset in pixel where the @src image should be blended