register: Add description below name field.

This commit adds description text below the name field
mentioning where the field will be used.

Fixes part of #29226.

templates/zerver/new_user_email_address_visibility.html

@@ -1,4 +1,4 @@
-<p id="new-user-email-address-visibility">
+<p id="new-user-email-address-visibility" class="registration-form-hint">
     <input type="hidden" name="email_address_visibility" id="email_address_visibility"/>
     <span class="current-selected-option">
         {% if default_email_address_visibility == email_address_visibility_admins_only %}

templates/zerver/register.html

@@ -124,6 +124,7 @@ Form is validated both client-side using jquery-validation (see signup.js) and s
                             {% endfor %}
                         {% endif %}
                     {% endif %}
+                    <p class="registration-form-hint">{{ _('This is how your account is displayed in Zulip.') }}</p>
                 </div>
 
                 <div class="input-box" id="profile_info_section" style="display:none;">

web/styles/portico/portico_signin.css

@@ -668,7 +668,7 @@ html {
     }
 }
 
-#new-user-email-address-visibility {
+.registration-form-hint {
     text-align: left;
     font-size: 0.8em;
     line-height: normal;