gh-86726: Add few missing versionadded directives (GH-151662)

Pack.pack_content, Place.place_content and Grid.grid_content were
added in 3.15.

Author: serhiy-storchaka

Doc/library/tkinter.rst

@@ -2979,6 +2979,8 @@ Base and mixin classes
       Same as :meth:`Misc.pack_content`.
       :meth:`content` is an alias of :meth:`!pack_content`.
 
+      .. versionadded:: 3.15
+
 
 .. class:: Place()
 
@@ -3073,6 +3075,8 @@ Base and mixin classes
       Same as :meth:`Misc.place_content`.
       :meth:`content` is an alias of :meth:`!place_content`.
 
+      .. versionadded:: 3.15
+
 
 .. class:: Grid()
 
@@ -3221,6 +3225,8 @@ Base and mixin classes
       Same as :meth:`Misc.grid_content`.
       :meth:`content` is an alias of :meth:`!grid_content`.
 
+      .. versionadded:: 3.15
+
 
 .. class:: XView()