Improve 'Types and Specs' introduction

[ctstacey]

Clarifies the statement "all types in Elixir are checked at runtime". They are not checked per say, exceptions are thrown. Thus I think it's better to say "Types are not checked at compile time".

Expands on what Types and Specs are useful for (i.e. documentation, code clarity, static code analysis).

[josevalim]

They are actually checked at runtime. Every time we do an operation, such as addition, we check the type and raise if they are invalid. You get exceptions as a consequence of the check :)

[ctstacey]

Thanks Jose, fair comment. Is there anything in the second paragraph you feel worth pulling, or shall I close the PR?

[ctstacey]

Thanks for considering my PR and for the feedback 👍 😃

[josevalim]

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