RFR: 8374953: Add note on about implicit state when comparing TypeMirrors

[Joe Darcy]

The API note about comparing Element's:

> The identity of an element involves implicit state not directly accessible from the element's methods, including state about the presence of unrelated types. Element objects created by different implementations of these interfaces should not be expected to be equal even if "the same" element is being modeled; this is analogous to the inequality of Class objects for the same class file loaded through different class loaders.

is equally applicable to comparing TypeMirror and an analagous note should be present on the methods used to compare TypeMirror's.

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Commit messages:
 - Update copyright year.
 - JDK-8374953: Add note on about implicit state when comparing TypeMirrors

Changes: https://git.openjdk.org/jdk/pull/29153/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=29153&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8374953
  Stats: 20 lines in 2 files changed: 18 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/29153.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/29153/head:pull/29153

PR: https://git.openjdk.org/jdk/pull/29153