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

[Drew Adams]

The doc string is less clear in 27.1 than it was in 26.3.

Instead of saying that a generalized variable is involved, it says:

  You can use `alist-get' in PLACE expressions.  This will modify
  an existing association (more precisely, the first one if
  multiple exist), or add a new element to the beginning of ALIST,
  destructively modifying the list stored in ALIST.

A user can a least look up "generalized variable" in Emacs 26.  The
Emacs 27 version just refers to "PLACE expressions", which is unclear
and can't be looked up easily.  And why is PLACE uppercase, which
indicates that it's something in the calling sequence or is otherwise
defined in the doc string somehow?

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?