doc threads concurrency

[Kevin Ryde]

This attempts to describe the situation in the current head,

        * scheme-scheduling.texi (Threads): Note Guile uses POSIX threads, for
        concurrency and preemption.
        (C level thread interface): Note periodic libguile call required for C
        code in threads, add commented out reminders for SCM_TICK and
        guile-mode, for when those features are ready.

Incidentally, I'd be a bit tempted at some stage to have mutexes and
condition variables in sections of their own, with scheme and C
described together.  Which might leave "C level thread interface" as
just the C level thread creation and rules for operating.

scheme-scheduling.texi.threads.diff
Description: Text document