Add documentation to describe including multiple subprojects with a release

Submitted by Wayne Beaton @wbeaton

Link to original bug (#495692)

Description

When a project chooses to include subprojects with a release, we have a means of capturing that in the release record. This information is used currently as a hint to the IP Log generator to include those subprojects when it generates.

We should provide documentation of this in both the section on using the PMI to capture release information and as a FAQ entry in the IP Log generator section.

This impacts relatively few projects (Eclipse, Data Tools, Mylyn, and Web Tools), but is still important information.