doc: Mention timezone(zone, time) in section for AT TIME ZONE

doc: Mention timezone(zone, time) in section for AT TIME ZONE

timezone(zone, timestamp) is already mentioned as an equivalent of the
two first patterns in the table describing the AT TIME ZONE variants,
but did not mention the third case about "time" and its equivalent as an
SQL function, so let's be consistent here.

Extracted from a larger patch by the same author.

Author: Vik Fearing
Discussion: https://postgr.es/m/8e25dec4-5667-c1a5-6581-167d710c2182@postgresfriends.org