DDS12 — PIM description of "get_domain_id" method is missing

Summary:
In section 2.1.2.2.2, the "get_domain_id" method is mentioned in the table, but is not explained in the following sections.

Proposed Resolution::
Add a section that explains the "get_domain_id" method.

Proposed Revised Text::
Replace section 2.1.2.2.1.26 with the following one:

2.1.2.2.1.26 get_domain_id
This operation retrieves the domain_id used to create the DomainParticipant. The domain_id identifies the Domain to which the DomainParticipant belongs. As described in the introduction to Section 2.1.2.2.1 each Domain represents a separate data "communication plane" isolated from other domains.

Add a section that explains the "get_domain_id" method