Rollup merge of #131339 - HeroicKatora:set_ptr_value-documentation, r…

…=Mark-Simulacrum

Expand set_ptr_value / with_metadata_of docs

In preparation of a potential FCP, intends to clean up and expand the documentation of this operation.

Rewrite these blobs to explicitly mention the case of a sized operand. The previous made that seem wrong instead of emphasizing it is nothing but a simple cast. Instead, the explanation now emphasizes that the address portion of the argument, together with its provenance, is discarded which previously had to be inferred by the reader. Then an example demonstrates a simple line of incorrect usage based on this idea of provenance.

Tracking issue: rust-lang/rust#75091