docs(windows): Add note on split user/admin accounts

If the computer has split user/admin accounts, then language association may not be completed automatically, and the user may need to add a language association themselves.

Ref https://community.software.sil.org/t/what-security-permissions-are-needed-to-install-a-keyboard-configuration-in-keyman/5695/10

Note: the topic `configure-computer` has a terrible url but I won't fix that here. Suggested future fix -- turn `configure-computer` into a redirect to `link-keyboard-to-language` or `associate-keyboard-with-language`?

@keymanapp-test-bot skip