Fix pointer-to-paramref syntax in docs

The documentation for GLib and Gio contains a number of documentation
comment blocks where a parameter reference (e.g. `@err`) is also
discussed in its dereferenced pointer form, which is generally
annotated as `*@err`. This inevitably confuses the MarkDown parser,
which sees the `*` as the beginning of an italicized text span.

To avoid this, replace all `*@foo` with <code>`*foo`</code>, which
loses the `@`-sigil linking it to the `@foo` parameter, but formats
correctly in the rendered documentation. (`@foo` is automatically
formatted like <code>`foo`</code>, so the resulting appearance in
the docs is as intended.)

gio/gdbuserror.c

@@ -597,7 +597,7 @@ g_dbus_error_new_for_dbus_error (const gchar *dbus_error_name,
  * @format: (nullable): printf()-style format to prepend to @dbus_error_message or %NULL.
  * @...: Arguments for @format.
  *
- * Does nothing if @error is %NULL. Otherwise sets *@error to
+ * Does nothing if @error is %NULL. Otherwise sets `*error` to
  * a new #GError created with g_dbus_error_new_for_dbus_error()
  * with @dbus_error_message prepend with @format (unless %NULL).
  *