bug#40213: what-cursor-position could tell the meaning of marks too!

[Juri Linkov]

>> If a mode puts some sort of special marker in a buffer, it can
>> put that text property on it.
>
> But that would be slow.  The use case here is all the different
> characters used as markers in a Gnus Summary buffer -- and there may be
> tens of thousands of them.  Making generation of these buffers (which is
> already slow) even slower, just to make it possible to these characters
> for their meaning, would be a disservice to the users.

Maybe then 'describe-char' could check if the mode's symbol has a special 
property
with a function name, and output a string returned from that function, e.g.

  (put 'gnus-summary-mode 'describe-char 'gnus-summary-mode-describe-char)

Then 'describe-char' called in a gnus-summary-mode buffer will get the
function from its property, and 'gnus-summary-mode-describe-char' will
introspect its buffer and report information about the character
displayed at the given position and its meaning.