RFR: 4517644: Core reflection setter type conversion spec bugs

[Chen Liang]

### Current Status
In core reflection, there are a few holes with the documentation on the type conversions performed on the different setters. They generally follow this process:

1. The Object-valued setters checks the destination type. If that type is primitive, unbox the object.
2. Perform identity or (primitive/reference, depending on destination type) widening conversions.

For each step, they can fail with IllegalArgumentException, which has been the long standing behavior.

Note that this process is more restrictive than the JLS 5.2 assignment context, which allows boxing conversions, while the primitive reflective setters consistently reject them.

### Problems
There are some problems with current specs:
1. No mention that boxing is never done for primitive-typed setters
2. Array.set missing description for the final identity or widening conversion (reference or primitive) and the associated IAE condition
3. Field.set misses the identity or widening in IAE clause (but mentioned in main body)
4. Field primitive setters incorrectly claim they are equivalent to `set(instance, wrapper)` which is wrong due to lack of boxing conversion
5. Field primitive setters refer to nonsense "unwrapping conversion"

### Solutions
1. Make sure the unbox -> identity/widen process and the IAE conditoins are present in both Field and Array.set
2. Add that boxing is absent for all primitive setters
3. Fix the Field primitive setters' "unwrapping conversion" to be "identity or primitive widening conversion" as in Array primitive setters
4. Qualify the Field primitive setters assertion with "if this field is of a primitive type" to make it correct

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Commit messages:
 - 4517644: (reflect spec) Array.set documentation hole for array/value ref type mismatch

Changes: https://git.openjdk.org/jdk/pull/28029/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=28029&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-4517644
  Stats: 76 lines in 2 files changed: 20 ins; 0 del; 56 mod
  Patch: https://git.openjdk.org/jdk/pull/28029.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/28029/head:pull/28029

PR: https://git.openjdk.org/jdk/pull/28029