bug#58513: describe-function should say that defconst variables are constant

[Lars Ingebrigtsen]

Stefan Monnier <monnier@iro.umontreal.ca> writes:

> FWIW, my own local Emacs has `defconst` create actually
> constant variables by adding a `SYMBOL_DEFCONST` to `enum
> symbol_trapped_write`.  Admittedly I didn't make them really constant:
> I allow a subsequent `defconst` to change the value set by a previous
> `defconst`, but `setq` signals an error just like it does if you try to
> `setq` on `enable-multibyte-characters` or on `nil`.

Hm...  do you know if there's a performance impact from doing this?