ini_get: Update example to use ini_parse_quantity()

[edorian]

• Use ini_parse_quantity instead of custom code for ini value parsing.
• Replace register_globals ini value with something more current, as register_globals was removed in, I think, PHP 5.4.
• Change the wording for the ini_parse_quantity example to not contain “similar to” as it's deterministic output.

Resolves #4185

[TimWolla]

Created this PR together with edorian to showcase how editing the docs works. Requesting another review for wording approval.

[Girgias]

Thank you!