Doc: clarify use of RECURSIVE in WITH.
Apparently some people misinterpreted the syntax as being that
RECURSIVE is a prefix of individual WITH queries. It's a modifier
for the WITH clause as a whole, so state that more clearly.
Discussion: https://postgr.es/m/
ca53c6ce-a0c6-b14a-a8e3-
162f0b2cc119@a-kretschmer.de