Add fwd ref into CF Manual

[alexvas]

An intro of "Nullness Checker" section will reference "Generics and polymorphism" section to mention that defaults in the latter differs from defaults for a non-generic parameter, e.g. a Method argument.

That should help a new reader to catch the difference sooner. resolves: #795

[wmdietl]

Thanks for the suggestion!
I've started #804 with an extra reference and different wording and a few other typo fixes.
What do you think of that wording?

For the future, please make sure that your git authorship is set correctly.
CI failed because you are missing from the contributors.tex file.
With your next PR, please add yourself to that file.

However, note that the CI system determined two names for your commits:

Akash Srivastava,
+Aleksandr Vasilev,
 Alex Liu,
+Alexander Vasiljev,
 Alvin Abdagic,
 Anant Jain,

So please fix that in your setup.

Thanks for the PR and please keep suggestions coming!

[alexvas]

That wording is great, thank you.