docs: clarify Signal::derive() behavior

[gbj]

@benwis No rush -- Does this seem like a reasonable way to add context to Signal::derive()?

[benwis]

I like it, a lot clearer verbiage

[tqwewe]

Very clear! Definitely an improvement that would clear the initial confusion I had.

The (and convert it into a Signal<_>) part seems a little redundant but definitely just a nit pick

[bicarlsen]

There's two small typos on line 101

/// Values can be access with the same function call ...

Should be

/// Values can be accessed with the same function calls ...

I've added these as review comments.