Reword type assertions with events

[skirtles-code]

This change is inspired by the suggestion in #2174.

The use of the word 'cast' does seem to match how it is commonly used in a TypeScript context. However, it does have a slightly different meaning in other languages, and the TS docs seem to stay clear of using that word.

However, the proposal in #2174 to change it to the word 'assert' seemed less natural to me, even if it could be seen as more technically correct.

Instead, I've tried rewording the whole sentence, aiming to make it sound more like something from the TS docs.

The same sentence appears in both pages about TS usage, so I've applied the same change to both.

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