ShouldRetryForce should be settable by the event handler

The value was previously protected, but this actually stops whomever is
handling an even that receives the message from indicating that the
force calculation should be retried.
Documenting TargetSpeed change from df44307 on change log
Clarifying SteerForNeighbors subclasses should not be enabled manually
Expanding documentation of SteerForNeighborGroup properties
Expanded comments on SteerToFollowPath
BUGFIX on SteerForPathSimplified's minimum speed calculation

For some reason there was leftover code adding the prediction time to
the minimum speed to consider, if the vehicle was moving slower than the
minimum speed.
Merge branch 'release/3.0.0-alpha-2'