Add context for "Back to %s" string

[SergeyBiryukov]

Since [34202], the '← Back to %s' string is used in two different contexts:

• In wp-login.php, to return to the front end.
• In wp-admin/edit-tag-form.php, to return to the taxonomy screen.

In Russian, the first instance was translated as "Back to the site %s", and I'd prefer to keep it that way for clarity, so I think we should add a context to these strings:

• _x( '← Back to %s', 'site' )
• _x( '← Back to %s', 'admin screen' )

[SergeyBiryukov]

In 37703:

I18N: Add context and translator comments to Back to %s strings.

Fixes #37095.