revised wxString docs: don't use old style grouping of functions at the beginning...

revised wxString docs: don't use old style grouping of functions at the beginning of the file; rather use doxygen-style member groups; to be able to reference member groups, introduce the new @member_group_name and @ref_member_group ALIASES; fix some typos in the process

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@59623 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775