lavfi: fix mp and mandelbrot descriptions to make them complete sentences

This is consistent with the other filter descriptions.

libavfilter/vf_mp.c

@@ -878,7 +878,7 @@ static void end_frame(AVFilterLink *inlink)
 
 AVFilter avfilter_vf_mp = {
     .name      = "mp",
-    .description = NULL_IF_CONFIG_SMALL("libmpcodecs wrapper."),
+    .description = NULL_IF_CONFIG_SMALL("Apply a libmpcodecs filter to the input video."),
     .init = init,
     .priv_size = sizeof(MPContext),
     .query_formats = query_formats,

libavfilter/vsrc_mandelbrot.c

@@ -411,7 +411,7 @@ static int request_frame(AVFilterLink *link)
 
 AVFilter avfilter_vsrc_mandelbrot = {
     .name        = "mandelbrot",
-    .description = NULL_IF_CONFIG_SMALL("Mandelbrot fractal renderer."),
+    .description = NULL_IF_CONFIG_SMALL("Render a Mandelbrot fractal."),
 
     .priv_size = sizeof(MBContext),
     .init      = init,