bug#73886: 29.4; Confusing info about void function cells in Emacs Lisp manual

[Stefan Monnier via "Bug reports for GNU Emacs, the Swiss army knife of text editors" <bug-gnu-emacs@gnu.org>]

>> Oops.  Looks like I missed this part when I changed it back around
>> Emacs-24.4:
>> 
>>     ** In 'symbol-function', nil and "unbound" are indistinguishable.
>>     'symbol-function' does not signal a 'void-function' error any more.
>>     To determine if a symbol's function definition is void, use 'fboundp'.
>
> Could you explain the rationale for that change?  I tried to look for
> relevant discussions around that date, but came up empty-handed.

I can't remember discussing it, no.  It was a kind of "executive
decision".

Having a special "void" (`Qundefined`) non-value for the `symbol-value`
is needed for `boundp` since variables can contain *any* value, but not
for the `symbol-function` part where we can use any normal value (I
chose `nil`) to play this role as long as it doesn't collide with values
normally held in the `symbol-function` slot, like function names,
function values, cons cells, vectors, ...

The upside was a simplification in various chunks of code which used to
do things like `(and (fboundp SYM) (symbol-function SYM))` which can now
be simplified to `(symbol-function SYM)`.

I remember two "motivators", i.e. places where the need to pay attention
to the special void case annoyed me enough to look into this and make
the change, one was `nadvice.el` and the other was `cl-letf`.
[ So, it was no accident that the change happened in the same release as
  the addition of `nadvice.el`.  ]

In both cases the issue is that we want to deal with "places"
(generalized variables) and that abstraction works well for those places
which *always* contain a value, but not as well for those special places
that can be "unbound", so removing the "unbound" case from
`symbol-function` resulted in a welcome simplification.
For the same reason I dislike EIEIO's notion of `slot-boundp` and have
already considered marking it obsolete.



        Stefan