Several corrections to wxDocManager fields documentation.

Several corrections to wxDocManager fields documentation.

Remove non-existent wxDocManager::m_flags.

Added correct m_lastDirectory declaration to go with its documentation instead
of the duplicated m_fileHistory documentation.

Better document the default m_maxDocsOpen value.

Put all the documented fields in protected section as they're really protected
and not public.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@68333 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775