bug#35250: Add simplest Advice example possible first

[積丹尼 Dan Jacobson]

(info "(elisp) Advising Functions") starts out with a complicated
example.

        For example, in order to trace the calls to the process filter of a
     process PROC, you could use:

          (defun my-tracing-function (proc string)
            (message "Proc %S received %S" proc string))

          (add-function :before (process-filter PROC) #'my-tracing-function)

Please first add the simplest example possible:

Function A prints "a".

Make it print "ab" from now on.