apply PGP key box changes to registration page

placeholder changes and make command copyable. the command always uses
an example email address as the email address, because the email address
is unknown unless you fill in the email address field and mess up the
PGP key field.

metasrht/templates/register-step2.html

@@ -103,8 +103,16 @@
             name="pgpKey"
             style="font-family: monospace"
             rows="5"
-            placeholder="gpg --armor --export-options export-minimal --export fingerprint…"
+            placeholder="-----BEGIN PGP PUBLIC KEY BLOCK-----
+[...]
+-----END PGP PUBLIC KEY BLOCK-----"
           >{{pgpKey or ""}}</textarea>
+          <div id="gpg-command-section" class="form-text">
+            <p>
+              You can use the following command to export your PGP key (replace example@example.com with your email address):<br>
+              <code id="gpg-command">gpg --armor --export-options export-minimal --export example@example.com</code>
+            </p>
+          </div>
           <small class="form-text">
             Emails sent from {{cfg("sr.ht", "site-name")}} are
             signed with our PGP key:<br />