The `+m' flag is a long-overdue shorthand for `++xref-in setup/xref
load-collections-xref', which links to installed documentation in
the same way as DrRacket's "Scribble HTML" button.
That is, use `+m' to link to installed documentation,
scribble +m mine.scrbl
instead of the previously recommended
scribble ++xref-in setup/xref load-collections-xref mine.scrbl
Merge to 5.3.2