Update the documentation for ImplInitGraphics()

Better explain the whys and wherefores of the graphics LRU and
how it is used.

Change-Id: I345cd91044bc533c9650e2d17bf53d2879af3e39
Reviewed-on: https://gerrit.libreoffice.org/8030
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>