Clarify the TESTFN argument to `alist-get'

* lisp/subr.el (alist-get):
Rephrase the initial text to clarify the meaning of the TESTFN argument.
It's an equality predicate, not a look-up function (Bug#35206).

(cherry picked from commit c8146558)
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Status Job ID Name Coverage
  Test
passed #1287
test

00:38:10