This documents the behavior introduced in vercel#62856 to allow globally
overriding the router cache invalidation period.

`static` and `dynamic` in this case refer to variable levels of
"liveness" and are unrelated to whether the segment itself is opting
into static or dynamic rendering. In other words, the current `dynamic`
default of 30s suggests that we consider the 30s delta to feel dynamic
while still providing some caching for performance reasons. Others might
prefer a value of 0s here, hence the configuration.

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Closes NEXT-2995