Expand and organize offset_of! documentation.

[kpreid]

• Give example of how to get the offset of an unsized tail field (prompted by discussion Expand CloneToUninit documentation. #133055 (comment)).
• Specify the return type.
• Add section headings.
• Reduce “Visibility is respected…”, to a single sentence.
• Move offset_of_enum documentation to unstable book (with link to it).
• Add offset_of_slice documentation in unstable book.

r? Mark-Simulacrum