Shorten warning about hash creation

doc/src/sgml/ref/create_index.sgml

src/backend/commands/indexcmds.c

src/test/regress/expected/create_index.out

src/test/regress/expected/enum.out

src/test/regress/expected/macaddr.out

src/test/regress/expected/replica_identity.out

src/test/regress/expected/uuid.out