doc monitor

[Kevin Ryde]

        * scheme-scheduling.texi (Higher level thread procedures): In monitor,
        don't let "newly created" suggest a mutex created on every evaluation.
        Note what "monitor" means.


 - macro: monitor body...
     Evaluate BODY, with a mutex locked so only one thread can execute
     that code at any one time.  Each `monitor' form has its own
     private mutex and the locking is done per `with-mutex' above.  The
     return value is the return from the last form in BODY.

     The term "monitor" comes from operating system theory, where it
     means a particular bit of code managing access to some resource and
     which only ever executes on behalf one one process at any one time.