Add note about statically known maps keys and types tracking across function calls to the changelog

[michallepicki]

When reading the 1.17 changelog, in my opinion people less familiar with how typecheckers work can read too much into the promised improvements, and they might be disappointed afterwards when trying out the typechecker. For example, these two points especially:

Elixir v1.17 will emit the following warnings at compile-time:

• Pattern matching against a map or a struct that does not have the given key, such as %{adress: ...} = user (notice address vs adress)
• Updating a struct or a map that does not define the given key, such as %{user | adress: ...}

mention maps in general, but if I understand correctly this will only work for maps in limited situations when the map type with its keys is statically known, e.g. when updating a map literal defined in the same function.

I think it's better not to over-promise, so I would propose adding some kind of caveat / info mention about the current general limitations of how the typechecker works. This wording and the caveat placement is probably not very good, but I wanted to open the PR to illustrate what I meant

[josevalim]

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