Properly deprecate $network_id argument in get_blog_count()

[jeremyfelt]

Until 3.1, get_blog_count() ran a query on the $wpdb->blogs table based on a site id. This behavior was deprecated when get_site_option() was implemented to return a cached version of the count.

Patch documents this and uses _deprecate_argument()

[SergeyBiryukov]

Should it be $site_id rather than $network_id? Looks like we use $site_id when referring to a network ID throughout the core, with populate_network() being the only exception.

[jeremyfelt]

I think $network_id as part of an effort to properly name going forward. See ​IRC conversation from earlier today.

[SergeyBiryukov]

In 25113:

Deprecate $network_id argument in get_blog_count(). props jeremyfelt. fixes #25129.