document the use of the strtmpl.cxx and remove unnecessary macro usage

Document the "calling" macros for the strtmpl.cxx template file.
And remove unnecessary use of those macros in the calling file.

Change-Id: I20e0dd74150773363b9fb557884b84692ce22f11
Reviewed-on: https://gerrit.libreoffice.org/6504
Reviewed-by: Michael Stahl <mstahl@redhat.com>
Tested-by: Michael Stahl <mstahl@redhat.com>