doc: Document new previous/next event action in CFV

doc: Document new previous/next event action in CFV

Document the "Go to next/previous event of the selected thread"
actions, and how they differ from the "Next/Previous State Change"
ones.

Change-Id: I27c1431933e1ac1e8ae9fe94090ab13e82b25a14
Signed-off-by: Alexandre Montplaisir <alexmonthy@efficios.com>
Reviewed-on: https://git.eclipse.org/r/80605
Reviewed-by: Patrick Tasse <patrick.tasse@gmail.com>
Tested-by: Patrick Tasse <patrick.tasse@gmail.com>
Reviewed-by: Hudson CI