Explain keyof before it first appears in a snippet

[natepmay]

In the Using Type Parameters in Generic Constraints section of the Generics chapter of the handbook, an example is introduced that uses keyof. However, the keyof operator isn't introduced until the next chapter. As a result, I was confused by the example as I was working through the handbook in sequence.

I've kept the example but briefly introduced keyof beforehand to make the example comprehensible.

[natepmay]

@microsoft-github-policy-service agree