[Foundation] Fix nullability in NSDictionary. (#24478)

This is file 42 of 47 files with nullability disabled in Foundation.

* Enabled nullable reference types.
* Made ObjectForKey return nullable TValue? since keys may not exist.
* Made TryGetValue parameter nullable with NotNullWhen(true) attribute.
* Kept IDictionary interface implementations non-nullable per .NET contract.
* Replaces 'To be added' XML comments with proper documentation.
* Verifies and improves XML comments for language, grammar, consistency and correctness.
* Adds see cref attributes for type references.
* Removes unnecessary whitespace in XML comments.
* Fixes typos ('pais' -> 'pairs').
* Updates System.Object references to use simpler 'object' syntax.

Contributes towards #17285.

Author: rolfbjarne