Add notes in Object about TranslationServer methods

doc/classes/Object.xml

@@ -1034,6 +1034,7 @@
  Translates a [param message], using the translation catalogs configured in the Project Settings. Further [param context] can be specified to help with the translation. Note that most [Control] nodes automatically translate their strings, so this method is mostly useful for formatted strings or custom drawn text.
  If [method can_translate_messages] is [code]false[/code], or no translation is available, this method returns the [param message] without changes. See [method set_message_translation].
  For detailed examples, see [url=$DOCS_URL/tutorials/i18n/internationalizing_games.html]Internationalizing games[/url].
+ [b]Note:[/b] This method can't be used without an [Object] instance, as it requires the [method can_translate_messages] method. To translate strings in a static context, use [method TranslationServer.translate].
  </description>
  </method>
  <method name="tr_n" qualifiers="const">
@@ -1048,6 +1049,7 @@
  The [param n] is the number, or amount, of the message's subject. It is used by the translation system to fetch the correct plural form for the current language.
  For detailed examples, see [url=$DOCS_URL/tutorials/i18n/localization_using_gettext.html]Localization using gettext[/url].
  [b]Note:[/b] Negative and [float] numbers may not properly apply to some countable subjects. It's recommended to handle these cases with [method tr].
+ [b]Note:[/b] This method can't be used without an [Object] instance, as it requires the [method can_translate_messages] method. To translate strings in a static context, use [method TranslationServer.translate_plural].
  </description>
  </method>
  </methods>