Rephrase time-dependent part of path definition

Clarify the point raised by Dave Oran in #86 - Packets may traverse a different path, but any particular path itself does not change its composition. This is a strict path definition, and my changes are intended to clarify it, rather than change/add anything to the definition.

In the issue, Med brought up a loose path definition, as related to the Time-Variant Routing (TVR) proposed WG.

Upon further reflection, I wonder if this use case could be accomodated using our current path definition, e.g., if a path gets disrupted, it no longer exists as such (as no packets can be transmitted over it anymore), but a different path may have started to exist.

If this works, I would prefer to keep our path definition as-is, rather than adding a loose definition to it, as it is already long and complicated. If we have to add the loose definition, I wonder if we should confirm this with the Research Group.

panrg / path-properties

Rephrase time-dependent part of path definition #95