update terms of service instructions

We now want people to install the ToS file to /instance/static/terms-of-service.html in their Pleroma install instead of the previous location so upgrades don't wipe it.

static/terms-of-service.html

 <h4>Terms of Service</h4>
 
-<p>This is a placeholder ToS. Edit <code>"/static/terms-of-service.html"</code> to make it fit the needs of your instance.</p>
+<p>This is the default placeholder ToS.  You should copy it over to your static folder and edit it to fit the needs of your instance.</p>
+
+<p>To do so, place a file at <code>"/instance/static/terms-of-service.html"</code> in your
+   Pleroma install containing the real ToS for your instance.</p>
+<p>See the <a href='https://docs.pleroma.social/backend/configuration/static_dir/'>Pleroma documentation</a> for more information.</p>
+<br>
 <img src="/static/logo.png" style="display: block; margin: auto; max-width: 100%; height: 50px; object-fit: contain;" />

[HJ]

should we just replace whole pointer to a link "static dir" that leads to https://docs-develop.pleroma.social/backend/configuration/static_dir/ ?