ptr::RawPtr, spell out units used for the `offset` argument.

spell out units used for the `offset` argument, so that callers do not
try to scale to byte units themselves.
auto merge of #12420 : pnkfelix/rust/fsk-improve-doc-for-ptr-offset, r=alexcrichton

ptr::RawPtr, spell out units used for the `offset` argument.

spell out units used for the `offset` argument, so that callers do not
try to scale to byte units themselves.

(this was originally landed in PR #11002 for the stand-alone functions, but that PR did not modify the `RawPtr` methods, since that had no doc at all at the time.  Now `RawPtr` has the *only* documentation for `offset`, since the stand-alone functions went away in PR #12167 / PR #12248.)