[plt-scheme] Minor scribble reader change

In svn I have changed the scribble reader a little.  Before the
change,

  @list{... @foo at bar ...}  --reads-as-->  (list "... " foo bar " ...")

This was asymmetric to:

  (list @foo at bar)  --reads-as-->  (list foo at bar)

So after the change that I committed:

  @list{... @foo at bar ...}  --reads-as-->  (list "... " foo at bar " ...")

The bottom line is that if you had something like this in your
documentation:

  blah blah @x at y blah blah

then you now need to use either:

  blah blah @|x|@y blah blah

or even better:

  blah blah @|x y| blah blah

FWIW, I found this regexp pattern:

  @[a-z0-9A-Z!%^&*_+=.<>/?-]+@

useful in finding places that need to be fixed.

(And I did fix all occurrences in the PLT tree, and verified that I
covered everything using `read' before and after the change on all
files, and then I compared the results.)

-- 
          ((lambda (x) (x x)) (lambda (x) (x x)))          Eli Barzilay:
                    http://barzilay.org/                   Maze is Life!