bug#43836: 27.1; Doc string of `alist-get'

[Drew Adams]

> > It's also not clear how _adding_ a new element to the beginning of ALIST
> > destructively modifies it.  Does it really mean setting the car of ALIST
> > to a different element, not adding an element?
> 
> The example that follows on clarifies what's meant here.

Not really.  Not clear to me what's meant.

The first example adds (b . 2) to the front of the
alist.  OK.  But does it destructively modify the
list?  Or does `foo' just point to a new list (new
cons) whose cdr is the old list?

(I'm sure that some operations with `alist-get' do
modify list structure.  But I don't see how adding
an element to the front of the list does that.)

And how do you add an element (b . 3) to the front
of an alist that has an element (b . 2)?  An alist
can have multiple elements with the same key.

Sorry, but just what the behavior is for a place
expression that uses `alist-get' isn't clear to me
from that doc string, at all.

I have the same questions for the description at
(elisp) `Association Lists'.

This is apparently a useful function, and a bit
complex.  I suspect the doc isn't doing it justice,
but not understanding the actual behavior I can
only wonder.