doc: Improve description of min_dynamic_shared_memory
While on it, fix one oversight in
90fbf7c, that introduced a reference
to an incorrect value for the compression level of pg_dump.
Author: Justin Pryzby
Reviewed-by: Thomas Munro, Michael Paquier
Discussion: https://postgr.es/m/CA+hUKGJRTLWWPcQfjm_xaOk98M8aROK903X92O0x-4vLJPWrrA@mail.gmail.com