pgsql: doc: clarify that storage parameter values are optional

doc: clarify that storage parameter values are optional

In a few cases, the documented syntax specified storage parameter values
as required.

Reported-by: galiev_mr@taximaxim.ru

Discussion: /messages/by-id/159283163235.684.4482737698910467437@wrigleys.postgresql.org

Backpatch-through: 9.5

Branch
------
REL_13_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/0bddb3a95995008ed116858ddde9a89e01659dae

Modified Files
--------------
doc/src/sgml/ref/alter_index.sgml | 4 ++--
doc/src/sgml/ref/alter_materialized_view.sgml | 2 +-
doc/src/sgml/ref/alter_table.sgml | 4 ++--
doc/src/sgml/ref/create_index.sgml | 2 +-
4 files changed, 6 insertions(+), 6 deletions(-)