RFR: 8289434: x86_64: Improve comment on gen_continuation_enter()

[Richard Reingruber]

On Wed, 29 Jun 2022 08:23:36 GMT, Richard Reingruber <rrich at openjdk.org> wrote:

> Change code comments for `gen_continuation_enter()` explaining that the generated code will call `Continuation.enter(Continuation c, boolean isContinue)` if the continuation give as first parameter is run for the first time.
> 
> Also mention the special case for resolving this call.

Thanks for reviewing!

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PR: https://git.openjdk.org/jdk/pull/9320