[YSQL] [DOCS] Update the SQL syntax for the ysqlsh "\h [NAME]"

[stevebang]

One of the psql meta-commands, \h, returns command line help for SQL statement syntax. Currently, this is enabled in YugaByte DB in ysqlsh, but returns the help from PostgreSQL, not YugaByte DB. We should update this to reflect YSQL differences and extensions.

I'd also like us to include a link to the documentation page for details.

psql example:

postgres@postgres=#  \h alter table
Command:     ALTER TABLE
Description: change the definition of a table
Syntax:
ALTER TABLE [ IF EXISTS ] [ ONLY ] name [ * ]
    action [, ... ]
ALTER TABLE [ IF EXISTS ] [ ONLY ] name [ * ]
    RENAME [ COLUMN ] column_name TO new_column_name
ALTER TABLE [ IF EXISTS ] [ ONLY ] name [ * ]
    RENAME CONSTRAINT constraint_name TO new_constraint_name
ALTER TABLE [ IF EXISTS ] name
    RENAME TO new_name
ALTER TABLE [ IF EXISTS ] name
    SET SCHEMA new_schema
ALTER TABLE ALL IN TABLESPACE name [ OWNED BY role_name [, ... ] ]
    SET TABLESPACE new_tablespace [ NOWAIT ]
ALTER TABLE [ IF EXISTS ] name
    ATTACH PARTITION partition_name { FOR VALUES partition_bound_spec | DEFAULT }
ALTER TABLE [ IF EXISTS ] name
    DETACH PARTITION partition_name

where action is one of:

    ADD [ COLUMN ] [ IF NOT EXISTS ] column_name data_type [ COLLATE collation ] [
    DROP [ COLUMN ] [ IF EXISTS ] column_name [ RESTRICT | CASCADE ]