sw: document SwRectFnCollection instances

GCC would allow these to be non-comments according to
<https://gcc.gnu.org/onlinedocs/gcc/Designated-Inits.html#Designated-Inits>,
but still have them as comments for the sake of other compilers.

Change-Id: I4e615a41062edcc563b109753af8c2c457ee4503
Reviewed-on: https://gerrit.libreoffice.org/44443Reviewed-by: Miklos Vajna <vmiklos@collabora.co.uk>
Tested-by: Jenkins <ci@libreoffice.org>