replaygain: fix up doc links to defunct replaygain.org website

Fixes #624

gst/replaygain/gstrganalysis.c

@@ -26,7 +26,7 @@
  * @see_also: #GstRgVolume
  *
  * This element analyzes raw audio sample data in accordance with the proposed
- * <ulink url="http://replaygain.org">ReplayGain standard</ulink> for
+ * [ReplayGain standard](https://wiki.hydrogenaud.io/index.php?title=ReplayGain) for
  * calculating the ideal replay gain for music tracks and albums.  The element
  * is designed as a pass-through filter that never modifies any data.  As it
  * receives an EOS event, it finalizes the ongoing analysis and generates a tag
@@ -63,9 +63,8 @@
  *
  * ## Acknowledgements
  *
- * This element is based on code used in the <ulink
+ * This element is based on code used in the [vorbisgain](https://sjeng.org/vorbisgain.html)
- * url="http://sjeng.org/vorbisgain.html">vorbisgain</ulink> program and many
+ * program and many others.  The relevant parts are copyrighted by David Robinson, Glen Sawyer
- * others.  The relevant parts are copyrighted by David Robinson, Glen Sawyer
  * and Frank Klemm.
  *
  */
@@ -230,10 +229,8 @@ gst_rg_analysis_class_init (GstRgAnalysisClass * klass)
    * ReplayGain, set this property to 83.
    *
    * Almost all software uses 89 dB as a reference however, and this value has
-   * become the new official value.  That is to say, while the change has been
+   * become the new official value, and that change has been acclaimed by the
-   * acclaimed by the author of the ReplayGain proposal, the <ulink
+   * original author of the ReplayGain proposal.
-   * url="http://replaygain.org">webpage</ulink> is still outdated at the time
-   * of this writing.
    *
    * The value was changed because the original proposal recommends a default
    * pre-amp value of +6 dB for playback.  This seemed a bit odd, as it means

gst/replaygain/gstrglimiter.c

@@ -27,8 +27,8 @@
  *
  * This element applies signal compression/limiting to raw audio data.  It
  * performs strict hard limiting with soft-knee characteristics, using a
- * threshold of -6 dB.  This type of filter is mentioned in the proposed <ulink
+ * threshold of -6 dB.  This type of filter is mentioned in the proposed
- * url="http://replaygain.org">ReplayGain standard</ulink>.
+ * [ReplayGain standard](https://wiki.hydrogenaud.io/index.php?title=ReplayGain).
  *
  * ## Example launch line
  * |[

gst/replaygain/gstrgvolume.c

@@ -26,10 +26,11 @@
  * @see_also: #GstRgLimiter, #GstRgAnalysis
  *
  * This element applies volume changes to streams as lined out in the proposed
- * <ulink url="http://replaygain.org">ReplayGain standard</ulink>.  It
+ * [ReplayGain standard](https://wiki.hydrogenaud.io/index.php?title=ReplayGain).
- * interprets the ReplayGain meta data tags and carries out the adjustment (by
+ * It interprets the ReplayGain meta data tags and carries out the adjustment
- * using a volume element internally).  The relevant tags are:
+ * (by using a volume element internally).
  *
+ * The relevant tags are:
  * * #GST_TAG_TRACK_GAIN
  * * #GST_TAG_TRACK_PEAK
  * * #GST_TAG_ALBUM_GAIN
@@ -197,9 +198,7 @@ gst_rg_volume_class_init (GstRgVolumeClass * klass)
    * range.
    *
    * Note that the default value is 0 dB because the ReplayGain reference value
-   * was adjusted by +6 dB (from 83 to 89 dB).  At the time of this writing, the
+   * was adjusted by +6 dB (from 83 to 89 dB). The original proposal stated
-   * <ulink url="http://replaygain.org">webpage</ulink> is still outdated and
-   * does not reflect this change however.  Where the original proposal states
    * that a proper default pre-amp value is +6 dB, this translates to the used 0
    * dB.
    */