Improve docstrings for `Int`, `Float64`, `^`, and friends (#45221)
This extends the docs for `Int`, `Float64` etc, to
* Note that Int is the default, mention that it can overflow
* Explain what `1f0` means, and that Float64 is the default
Similarly extends docs for `^`, `+`, `*` aiming to
* Point out that `1.2 * 10^3` is a bad habit, warn about overflow
* Give an example for what `literal_pow` is doing since the explanation
is quite technical
* Also point out overflow in `+`, as [suggested
here](https://github.com/JuliaLang/julia/pull/45141#discussion_r862808888)
* While there, mention that you can add vectors, and that vararg
`+(1,2,3,4)` has a default binary behaviour
* Similarly mention that vararg `*(1,2,3,4)` has a default order, from
the left
* While there, mention `1/2pi` and `v'v` as examples of `*`.
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Co-authored-by: Lilith Orion Hafner <lilithhafner@gmail.com>