Add a link to the `$_SESSIONS` documentation from the documentation in `sessions.inc`.

[bugfolder]

This is a follow-on to an issue on the docs site (H/T @yorkshire-pudding):

https://github.com/backdrop-ops/docs.backdropcms.org/issues/212

We could address it by adding an @see link to the PHP predefined variables page, to clarify, where $_SESSION superglobal is mentioned in docblocks.

PR to follow.

[yorkshire-pudding]

Good idea @bugfolder

[klonos]

Indeed! Thanks @bugfolder and @yorkshire-pudding 🙏🏼

[quicksketch]

Thanks folks! Merged https://github.com/backdrop/backdrop/pull/4363 into 1.x and 1.24.x.