Do not pass `$site` and `$network` by reference in `get_*()` functions

[flixos90]

In get_site(), the $site parameter is passed by reference, which doesn't make a lot of sense. Same for the $network parameter in get_network(). Objects are passed by reference anyway, but passing for example the ID by reference could possibly lead to unexpected results.

[flixos90]

37615.diff​ adjust that. The patch also adjust a similar issue in update_site_cache(), and fixes the docs for the $network parameter of get_network() (previously didn't mention the default value).

[jeremyfelt]

Looks good. I don't think there's a reason for us to pass by reference here. I'm going to hold off on committing to trunk until we're good for 4.6 as well. Pinging @ocean90 for review. :)

[jeremyfelt]

In 38232:

Multisite: Remove unnecessary reference parameters.

There is no advantage to passing parameters by reference to get_site(), get_network(), and update_site_cache().

Props flixos90, ocean90 for review.
See #37615.

[jeremyfelt]

In 38233:

Multisite: Remove unnecessary reference parameters.

There is no advantage to passing parameters by reference to get_site(), get_network(), and update_site_cache().

Merge of [38232] to the 4.6 branch.

Props flixos90, ocean90 for review.
Fixes #37615.