8371795: Improve documentation of Class.isInstance

[liach]

The 3 methods to determine conversions and subtyping on java.lang.Class, which are isInstance, cast, and isAssignableFrom, have their documentation from the earliest days of the Java Platform. During the language evolution, a lot of terms have become inaccurate, such as "assignment-compatible", which does not apply for primitive types, and the out-of-date instanceof analogy with the upcoming patterns, in isInstance; isAssignableFrom is not very clear about arrays; cast would also benefit from more detailed explanations.

In my facelift, I moved the subtyping description to isAssignableFrom, and left the conversion stuff in isInstance and cast. I intentionally avoided linking to too many JLS chapters to reduce confusions. I believe in this shape, we have a good amount of easily comprehensible yet accurate specification for all 3 methods, and users are welcome to read the linked JLS chapters for more details and context.

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Progress

• Change must be properly reviewed (1 review required, with at least 1 Reviewer)
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• Commit message must refer to an issue
• Change requires CSR request JDK-8373192 to be approved

Issues

• JDK-8371795: Improve documentation of Class.isInstance (Bug - P4)
• JDK-8373192: Improve documentation of Class.isInstance (CSR)

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Webrevs

• 03: Full - Incremental (8dc4e31e)
• 02: Full - Incremental (5fd09962)
• 01: Full - Incremental (4a4f2867)
• 00: Full (3d5e795f)