Document Insert() and Prepend() overloads taking wxMenu.

Document these methods in addition to AppendSubMenu().

Closes #16052.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/branches/WX_3_0_BRANCH@76090 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775