common: document C_GatherBuilder and C_Gather

* Introduction explaining what they are used for
* Commented example to illustrate the abstract description

Signed-off-by: Loic Dachary <loic-201408@dachary.org>
Merge pull request #2609 from dachary/wip-doc-context

common: document C_GatherBuilder and C_Gather

Reviewed-by: John Spray <john.spray@redhat.com>