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

[Stefan Monnier]

>> Yes, incurring a run-time cost doesn't sound worth it, but we could
>> perhaps make the byte-compiler warn about it.

The byte-compiler already does warn about it, but only if the assignment
is in the same file as the `defconst`.

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`.

I didn't bother exporting this info to `describe-variable` (and the
byte-compiler) but it would be easy to do.


        Stefan