Clarify documentation for "instance?"

[Alan Grover]

Guile Documentation: Goops Manual, no version # in documentation
Guile Release: 1.6.4

The behavior of "instance?" needs clarification with regards to 
non-Goops entities.

There are many entities in Guile that can be used like a Goops object, 
but which aren't implemented as a Goops object. This allows native data 
types to be used in generic-procedure specialization, by giving them a 
class and class-hierarchy, etc. You can even ask almost everything for 
its class, via "class-of".

On the procedure side, there is a mechanism to allow many procedures to 
be specialized (see generic-capability?, for example).

However, "instance?" only returns true for a Goops object.

The documentation for "instance?", and perhaps in another section, 
should note the distinction between a Goops object and "things you can 
use like it". It should also give the definition of a Goops object (the 
result of "make"?).

--
Alan Grover
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+1.734.476.0969