sync/cond.go: add explanation comments to check func

[robinbraemer]

It adds two internal comments explaining why c is checked twice in a row and is intentional.

There has been confusion about why this was done, confusion should not occur anymore:

• sync/cond: redundant code in copyChecker.check #34516
• sync: document why copyChecker checks the condition twice #40924

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Patch Set 2:

(4 comments)

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surprising to people new to the project. The careful, iterative review process
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have a lasting impact.

During May-July and Nov-Jan the Go project is in a code freeze, during which
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