Explain how Vec::with_capacity is faithful

[workingjubilee]

There are concerns that the doc changes in 95dc353
are breaking changes in the promised API. In this commit, I explain in more detail
the exact promise that Vec::with_capacity is really making:
Vec is trying to act unsurprising by relaying information faithfully to the allocator,
it is not committing to internal details of Vec itself beyond that.
As it happens, we don't get useful capacity information from allocators,
but once upon a time Rust did capacity recalculation from available data,
and we should reserve the right to do so again if it seems profitable and correct.

This path avoids adding a duplicate with_capacity_exact to Vec's already-formidable API surface.

closes #99385.