add basic overview of when to use type declarations

[stevengj]

Questions about when to declare function arguments come up over and over on discourse, and so it seems useful to put a basic summary early in the chapter on Functions, rather than deferring it to later chapters.

[timholy]

This seems helpful and in the right place. There are other reasons to add types, but these are the most common and most relevant for newcomers.

[timholy]

Maybe slightly less emphasis, though? Feels a bit like shouting, though I understand why these points are worth emphasizing.

[stevengj]

With regard to bold/emphasized text, I was influenced a lot by Jakob Nielsen's usability studies, which suggests that online readers typically scan text rather than reading from top to bottom, and so it is useful to highlight key ideas in bold to aid scannability.

However, I can certainly cut down on this if people prefer; I've already updated it to trim the emphases somewhat. Let me know if you want me to go further.

[JeffBezanson]

Nice!