doc: clarify that storage parameter values are optional

Bruce Momjian <bruce@momjian.us>

Commit: aa90d9957bd2d9ce39f9078d15914a37d99af072
Author: Bruce Momjian <bruce@momjian.us>
Date: 2020-06-30T16:26:51Z
Releases: 14.0
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: https://postgr.es/m/159283163235.684.4482737698910467437@wrigleys.postgresql.org

Backpatch-through: 9.5

Files

Documentation touched