Fix some awkward wording in the `core::option` documentation in the
"Options and pointers" section.

Fix a typo/missed replacement in the documentation for
`impl From<&Option<T>> for Option<&T>` in `core::option`.

Fix an error in `map_or_else`. Use more descriptive text for
"don't care" in truth tables. Make minor corrections to truth tables.
Rename `makeiter` to `make_iter` in examples.

Make minor wording changes in a few places. Move `filter` to the
"transformations" section. Add `zip` methods to the "transformations"
section. Clarify the section about `Option` iterators, and add a section
about collecting into `Option`.

Clarify that for `Result`, `or` and `or_else` can also produce a
`Result` having a different type.

(Most of these are from a review by joshtriplett. Thanks!)

Fix errors in `as_pin_ref` and `as_pin_mut` in the "Adapters for
working with references" overview.

Reword some headings about transformation methods.

Reclassify `map`, `map_or`, `map_or_else`, `map_err`, etc. to more
accurately reflect which variants they transform.

Document `Debug` requirement for `get_or_insert_default`.

Reword text about `take` and `replace` to be more accurate.

Add examples for the `Product` and `Sum` traits.

Also:

Move link reference definitions closer to their uses. Warn about making
link reference definintions for `err` and `ok`. Avoid making other link
reference definitions that might conflict in the future (foreign methods
that share a name with local ones, etc.)

Write out the generics of `Option` and `Result` when the following
text refers to the type parameters.