Documentation on Streaming Replication interface for Logical Decoding

In section 46.3 (Streaming Replication Protocol Interface) the create command
for logical slots, CREATE_REPLICATION_SLOT, is defined as taking “options”
after the LOGICAL keyword when it in fact requires the plugin name.  The start
command is correctly listing the options but isn’t listing the required start
position.

The attached patch synchronizes these command signatures with the more detailed
descriptions in section 50.3* (Streaming Replication Protocol).

    cheers ./daniel

(*) Section number from the 9.5beta1 manual