Improve documentation of `align_of` and `Alignment`. * The documentation for `align_of_val()` and `Alignment::of_val()` did not explain its use in interacting with `dyn` types, and even contained the false statement that “Every reference to a value of the type `T` must be a multiple of this number”. This change removes that statement from everything except `align_of()`, and adds a mention of, and example code for, getting the alignment of a `dyn` value. * The documentation for `align_of_val_raw()` did not explain how it relates to other functions in the family. Now it does. * Added a caveat that the alignment of `i32` is not always 4, despite the examples asserting this. @rustbot label A-docs