nss: document why --with-system-nss can't be used in generic builds

Change-Id: I93ce16b4dd059ca0ed79975cfb6373dd629f29a9
fdo#80924: writerfilter: RTF import: fix shapeType PictureFrame import

For shapeType PictureFrame, a default CustomShape was created and then
RTFDocumentImpl::resolvePict() crashes because it actually operates on a
previous shape, because in this special case RTFSdrImport::m_xShape
is never actually set to the new shape, so contains the previous one.

Also the new shape needs to be added to the draw-page, at least
otherwise the assertion of the supported service fails because
some SvxShape::mpObj weak-reference is dead?

This essentially reverts commit 3cab1adf19d553663685e8198f0ec3f258a37c36
(except for the added testcase, which was slightly different because
it did not have a "pib" property).

(regression from ba9b63d8101197d3fd8612193b1ca188271dfc1a)

Change-Id: I6539c4286850dff2d8564006487cc765f1d117be