feat(custom-field): let admins set join_description with a link-syntax hint

lib/mv_web/live/custom_field_live/form_component.ex

@@ -91,6 +91,45 @@ defmodule MvWeb.CustomFieldLive.FormComponent do
           <% end %>
 
           <.input field={@form[:description]} type="text" label={gettext("Description")} />
+
+          <fieldset class="mb-2 fieldset">
+            <label>
+              <span class="mb-1 label flex items-center gap-2">
+                {gettext("Description for join form")}
+                <.tooltip
+                  content={
+                    gettext(
+                      "You can add links: full addresses (https://…) or as [link text](https://…)."
+                    )
+                  }
+                  position="right"
+                >
+                  <span
+                    data-testid="join-description-link-hint"
+                    aria-label={
+                      gettext(
+                        "You can add links: full addresses (https://…) or as [link text](https://…)."
+                      )
+                    }
+                  >
+                    <.icon
+                      name="hero-information-circle"
+                      class="w-4 h-4 text-base-content/60 cursor-help"
+                      aria-hidden="true"
+                    />
+                  </span>
+                </.tooltip>
+              </span>
+              <input
+                type="text"
+                name={@form[:join_description].name}
+                id={@form[:join_description].id}
+                value={Phoenix.HTML.Form.normalize_value("text", @form[:join_description].value)}
+                class="w-full input"
+              />
+            </label>
+          </fieldset>
+
           <.input field={@form[:required]} type="checkbox" label={gettext("Required")} />
           <.input
             field={@form[:show_in_overview]}