Mark retry# as returning bottom.

This change helps the simplifier eliminate unreachable code, since
retry# technically doesn't return.

This closes ticket #8091.

Authored-by: Patrick Palka <patrick@parcs.ath.cx>
Signed-off-by: Austin Seipp <aseipp@pobox.com>
Fix invalid users guide synax.

Signed-off-by: Austin Seipp <aseipp@pobox.com>