lavf/dashenc: require experimental mode to be enabled for WebM

It has not ever been working and has not been validated,
Additionally, mention that the segment file names should be changed
to end with webm instead of m4s, which is utilized for ISOBMFF
fragments.

libavformat/dashenc.c

@@ -966,6 +966,16 @@ static int dash_init(AVFormatContext *s)
             s->streams[i]->codecpar->codec_id == AV_CODEC_ID_OPUS ||
             s->streams[i]->codecpar->codec_id == AV_CODEC_ID_VORBIS) {
             snprintf(os->format_name, sizeof(os->format_name), "webm");
+
+            if (s->strict_std_compliance > FF_COMPLIANCE_EXPERIMENTAL) {
+                av_log(s, AV_LOG_ERROR,
+                       "WebM support in dashenc is experimental and has not "
+                       "been validated. For testing purposes, make sure "
+                       "to add -strict experimental and override "
+                       "-init_seg_name and -media_seg_name to end with "
+                       "the extension 'webm'.\n");
+                return AVERROR(EINVAL);
+            }
         } else {
             snprintf(os->format_name, sizeof(os->format_name), "mp4");
         }