rtl: starts-/endsWith* new second parameter since 4.2

Updated the documentation for the new optional second parameter in the
O(U)String startsWith* and endsWith* methods so it is explicitly said
that it is only available since LibreOffice 4.2

Change-Id: I58758e4bae85eef07c578dd50d6e0279b49deaf5
Reviewed-on: https://gerrit.libreoffice.org/6649
Reviewed-by: Stephan Bergmann <sbergman@redhat.com>
Tested-by: Stephan Bergmann <sbergman@redhat.com>