scribble: add `+m' / `++main-xref-in' flag

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