bug#24363: Clarify the VM docs

[Wilfred Hughes <me@wilfred.me.uk>]

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The docs currently state:

> The easiest (and most fun) way to depend on a virtual machine is to
implement the virtual machine within Guile itself. This way the virtual
machine provides what Scheme needs (tail calls, multiple values, call/cc)
and can provide optimized inline instructions for Guile (cons, struct-ref,
etc.).

This is a little confusing for new readers (like myself), who may consider
Guile to by synonymous with Scheme.

This led to interpret this as 'the VM is implemented in Scheme', which has
a bootstrapping problem (what actually interprets your bytecode?).

Instead, I propose adding an additional sentence:

> The easiest (and most fun) way to depend on a virtual machine is to
implement the virtual machine within Guile itself. Guile contains a
bytecode interpreter (written in C) and a Scheme to bytecode compiler
(written in Scheme). This way the virtual machine provides what Scheme
needs (tail calls, multiple values, @code{call/cc}) and can provide
optimized inline instructions for Guile (@code{cons}, @code{struct-ref},
etc.).

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