Change documented type of get_network_option, update_network_option and delete_network_option's $network_id parameter to int|null

[maikelraow]

get_network_option, update_network_option and delete_network_option all have a $network_id as first parameter.

@param int $network_id ID of the network. Can be null to default to the current network ID.

It's @param type is set to INT yet it also explicitly says that NULL can be passed to use the current network ID. If passing NULL is valid then should the type not be INT|NULL?

This PR updates the documented type of the $network_id parameter in get_network_option, update_network_option, and delete_network_option methods to int|null, reflecting that NULL can be passed to default to the current network ID.

I also confirmed that int|null has been used in other functions, such as get_blog_count, which internally calls get_network_option: ​https://github.com/WordPress/wordpress-develop/blob/dc9b16ba4224259bd3ae2153f81686ed902391f2/src/wp-includes/ms-functions.php#L103-L117

[audrasjb]

Hello and welcome to WordPress Core Trac @maikelraow,

Thanks for reporting this. Moving to milestone 6.8.

I approved this PR but I also spotted some other places that I'll fix directly when committing this changeset :)

[audrasjb]

In 59940:

Docs: Update $network_id parameter type to int|null in Network Option methods.

Props maikelraow, im3dabasia1, audrasjb, mukesh27.
Fixes #62735.
See #62281.