stbt.wait_until docstring: Use `stbt` prefix consistently
stbt.wait_until docstring: Use `stbt` prefix consistently

I like to import wait_until directly as it's a general utility function
and it feels more readable this way -- as opposed to functions like
`stbt.match` and `stbt.press` where the `stbt.` stands in for the
device-under-test.

stb-tester/stb-tester,stb-tester/stb-tester,stb-tester/stb-tester,stb-tester/stb-tester