Add randkeyfrom and randkeysfrom macros and their descriptions in help.php

[DaleWMyers]

Summary

Adds two new randomizer macros for MyOpenMath: randkeyfrom and randkeysfrom.
These mirror PHP's array_rand(), but pull from MOM's seeded random generator
($GLOBALS['RND']) so results stay reproducible for grading/regrading, consistent
with the rest of the macros in randomizers.php.

New functions

randkeyfrom($array)
Returns a single random key from an array. Equivalent to array_rand($array).

$arr = ["a"=>"A", "b"=>"B", "c"=>"C"];
$x = randkeyfrom($arr);  // e.g. $x = "b"

randkeysfrom($array, $n = 1)
Returns $n random keys from an array. Equivalent to array_rand($array, $n).
Unlike PHP's native array_rand(), which requires [$a, $b] = array_rand(...)
for multiple keys, randkeysfrom returns a plain array so it works with MOM's
existing multi-assignment syntax:

$arr = [0=>'x_0', 1=>'x_1', 2=>'x_2', 3=>'x_3'];
$a,$b = randkeysfrom($arr, 2);   // e.g. $a=2, $b=0
[$a,$b] = randkeysfrom($arr, 2); // e.g. $a=1, $b=3

randkeysfrom only accepts arrays (not comma-separated lists), since lists
have no keys — this differs from randfrom/randsfrom, which accept both.

Files changed

• assessment/macros/randomizers.php — function definitions, added to allowedmacros
• help.html — documentation entries added after the existing randfrom/randsfrom entries

Testing

Verified locally against QID 1141.