Read Committed Shared Lock
I can't find any documentation on this

Docs are here.  An S lock is compatible with a U lock, but the UPDATE will reuquire an X lock to actually complete, which is incompatible with an S lock.


and also don't know how to test this

You can always run a a query like
select count(*)
from sales.SalesOrderDetail with (tablock), master..spt_values v, master..spt_values v2, master..spt_values v3

And see if you can update from another session.  The tablock is to ensure that the whole table is locked with an S lock, as it may or may not be with a long-running SELECT.
You'll discover that unless the database has the READ COMMITTED SNAPSHOT option set, the SELECT will block the UPDATE.