bug#39233: .elc file - possibly outdated backward compatibility comments

[Stefan Monnier]

> In the .elc files on my machine, I see the following:
>     ;;; This file uses dynamic docstrings, first added in Emacs 19.29.

It's factually correct, but indeed it not really relevant, we can drop
this to save some bytes.

>     ;;; This file does not contain utf-8 non-ASCII characters,
>     ;;; and so can be loaded in Emacs versions earlier than 23.

Here, this is indeed likely not factually correct: some .elc files might
indeed work in earlier versions, but that will depend on many other
things:
- byte codes have been added in Emacs-24 for the lexical-binding
  support, others were added in Emacs-25, yet more in Emacs-26)
- macros which expand to code which relies on functions/features only
  provided in newer versions).
- changes in the reader (e.g. the readable syntax for hash-tables, or
  that of records).

I think more useful would be to emit a warning any time a .elc file is
loaded into an Emacs that's older than the one with which it
was compiled since that's a case we "don't support" (at least when the
major-version is different).

> 1. Is it still correct that the byte compiled file can be loaded on
>    Emacs 22?

It's quite likely that *some* do.  But I expect that the vast
majority don't.

> 3. Following up on point 2, I also see:
>    
>        "\n;;; This file uses "
>        (if dynamic-docstrings
>            "dynamic docstrings, first added in Emacs 19.29"
>          "opcodes that do not exist in Emacs 18")
>    
>    I'm not sure what to do here, but it doesn't seem to make sense to
>    mention Emacs 18 in any case.  Of course, this file might use
>    opcodes that do not exist in Emacs 23, or perhaps an even later
>    version.

Throw it all away.


        Stefan