docs: update `ThreadLocal<T>.asContextElement` documentation

[nikitabobko]

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The latest code snippet in the KDoc is no longer relevant. Coroutines update and restore the thread-local value on every suspension points.

The code became irrelevant after 727c38f708c355e325326def66c80dfec7f81ebe (related issue: https://github.com/Kotlin/kotlinx.coroutines/issues/985)

Use withContext to update the corresponding thread-local variable to a different value, for example:

The commit drops this paragraph and the attached code snippet. The snippet is useless since the correct usage pattern is already demonstraited in the first code snippet.