Improve filter type explanation in scc merge section

- Expose both forms of inclusion/exclusion by PR number
- Expose inclusion/exclusion of submodule PRs by number
Add section about scc travis-merge command

- Expose the comment parsing strategy to define the include filters
Pluralize filters as suggested by Mark
Replace Pull Request by its abbreviation
Merge pull request #477 from sbesson/travis-merge

Travis merge documentation