8353197: Document preconditions for JavaLangAccess methods

[vy]

Document preconditions on certain JavaLangAccess methods that use operations either unsafe and/or without range checks.

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• JDK-8353197: Document preconditions for JavaLangAccess methods (Enhancement - P3)

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[mlbridge]

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• 00: Full (ef88b10a)

[liach]

I think we should add a note on why no copy is made - there may be multiple reads to the input array for decoding, so the input array must not be accessible by untrusted code, which can racily modify it. A side effect is that this array is simply reused if it's eligible to be the string content array, which is unfortunately abused in a few places in the JDK.