changes made to documentation

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

I just wrote new versions of the functions and concepts files. 

The major changes I made include:

Concepts:

  - Simplified "Defining a Class"
  - Added section "Redefining a Class"
  - Rewrote Inheritance of Slots 

Functions:
 
  - Fixed description of arguments to DOCUMENTATION 

  - Made the syntax of options in DEFCLASS stand out better in
    the description of each option

  - Fleshed out DEFINE-METHOD-COMBINATION to include a description
    of each option.   Note that these are just one-liners, and the
    full syntax is still in the Concepts chapter, as are the examples.
    I think this is the best way we can describe this function.

  - Rewrote CHANGE-CLASS according to latest proposal

  - Added description of CLASS-CHANGED

  - Added several See Also fields.  For example, you wanted to discuss
    "compatibility  

  - Fleshed out the Introduction to mention what role each function and
    macro plays -- that is, I put them into categories.   This should 
    help people put the functions into a context.   People at Symbolics
    who read the drafts of this were put off by the alphabetic
    (contextless) order of functions,  so this should solve that
    problem.
 
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I also made the changes suggested in Moon's message.