Rollup merge of #52508 - joshtriplett:dangling-not-sentinel, r=Mark-Simulacrum

Document that Unique::empty() and NonNull::dangling() aren't sentinel values

The documentation of Unique::empty() and NonNull::dangling() could
potentially suggest that they work as sentinel values indicating a
not-yet-initialized pointer. However, they both declare a non-null
pointer equal to the alignment of the type, which could potentially
reference a valid value of that type (specifically, the first such valid
value in memory). Explicitly document that the return value of these
functions does not work as a sentinel value.