updated documentation

[Sonya E. Keene <skeene@STONY-BROOK.SCRC.Symbolics.COM>]

I wrote new versions of the functions, concepts, and design chapters.
The documentation now includes the write-up about Standard Type Classes
which I sent to this list several months ago.   I didn't receive any
comments about it, probably because the text was cluttered up with tex
formatting directives.   When we format the document again it will be
easier for people to read this section and make their comments then.  

I changed the documentation to state that the def-XXX macros return 
objects, not names.   We agreed on this change at our Cambridge meeting
a couple weeks ago.

The only problem with this is some awkwardness about the value of
define-method-combination.    There is no CLOS object for representing a
method combination type.   define-method-combination expands into a
defmethod, so its result is a method object.