Statistics: Clarify provider history introduction

themes/xmpp-providers/layouts/shortcodes/statistics.html

@@ -29,7 +29,7 @@ <h3 id="history-of-providers">
  History of Providers
  <a class="heading-anchor" href="#history-of-providers"><i class="fa-solid fa-link fa-xs"></i></a>
  </h3>
- <small class="text-muted">This chart shows since when providers are available or listed.</small>
+ <small class="text-muted">This chart shows since when providers are available. Providers are listed in "Unknown", if the date of going into sevice was not provided.</small>
  <div id="since_bar_chart_container" style="height:400px;"
  data-values="{{ .Site.Data.statistics.since_bar_chart_data | jsonify }}"></div>
  </div>