doco scm_remember_upto_1

[Kevin Ryde]

In the "Type checking" node of the manual showing the clear_image
example smob code, should that function have an scm_remember_upto_1
just before returning, in order to keep image_smob visible to a
garbage collect that might run in another thread?

I was going to write some words about scm_remember_upto_1.  I was
going to have a new "map_image" to illustrate the point, then wondered
whether clear_image ought already show it.  Either that or it's an
example of when not needed.

I'm assuming since scm_remember_upto_1 is mentioned in the 1.6 NEWS
that it's a documented (or meant to be documented) feature.