docs: clarify enter and stay causes for parameter changes

[LadyBluenotes]

The cause value passed to beforeLoad and loader is currently listed as enter, stay, or preload, but the docs do not explain what distinguishes enter from stay.

This is easy to misread when navigating between parameter values. For example, moving from /somepath/one to /somepath/hello%20world creates a different match ID, but the same route remains active, so the cause is stay.

It would help to clarify that:

• enter means the route ID has newly entered the active route tree.
• stay means the route ID remains active, including when path params or loader dependencies change.
• preload applies to preloading work.

That should make it clearer when to use cause for route lifecycle behavior versus match-specific behavior.

Related Work: #3660

[LadyBluenotes]

Adding a concrete example from #3660: a fresh load of /somepath/hello%20world should use enter. Once /somepath/$param is already active, navigating between parameter values uses stay, even though the match ID changes.

Including an encoded parameter example would make the route-ID-based distinction less surprising.