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
Uses "Github flavored markdown". Specifically, code blocks are opened
using ```scheme so that Github will lex and format them as Scheme code
rather than generic monospace.
Note: I would have used ```racket, but we are still waiting for the
pygments.rb project to pull again from pygments-main -- to which I
contributed a Racket lexer back in August. After pygments.rb pulls,
can update this to use ```racket instead.
