Document indexOf return value when not found

[Pascal-So]

fixes #60570

The documentation for indexOf on arrays already mentioned that the method returns -1 when the value is not found:

But for strings, this information was missing thus far:

Of course the user would have been able to puzzle this together by seeing that the return type is just number, not something like number | undefined, thus allowing them to figure out that the sentinel value probably must be -1. Nevertheless, having to stop and think about this is unnecessary friction for people who don't remember this fact by heart, for example people who jump between programming languages a lot.

I also noticed that the same information was missing for all the TypedArrays, so I went ahead and extended the docs there as well.

[typescript-bot]

This PR doesn't have any linked issues. Please open an issue that references this PR. From there we can discuss and prioritise.

[Pascal-So]

@microsoft-github-policy-service agree