Update @function documentation for css calc function changes

[ntkme]

Since 1.65.0 pow() is always parsed as calculation instead of custom function. If user defines a custom function with the same name as native css function, Sass would not parse it as function any more.

It means the example that defines a custom pow() function is no longer correct and need to be updated: https://sass-lang.com/documentation/at-rules/function/

@function pow($base, $exponent) {
  $result: 1;
  @for $_ from 1 through $exponent {
    $result: $result * $base;
  }
  @return $result;
}

// This example still produce the same output, but the custom function is not called due to `pow(4, 3)` being parsed as calc.
.sidebar {
  float: left;
  margin-left: pow(4, 3) * 1px;
}

Also, it is probably worth to have a breaking change notice as some users got bite by this behavior change:

• [email protected] breaks optional parameters for functions? dart-sass#2059
• @1.49.9 Skipped not serializable cache item 'Compilation/modules dart-sass#2060

@pamelalozano16