RFR: JDK-8272945: Use snippets in java.compiler documentation

[Jonathan Gibbons]

Please review a patch to use snippets in the `java.compiler` documentation, instead of a mix of raw HTML and/or `{@code ...}`.  The change is just about the presentation of the code fragments; there are no changes to the normative specifications for the module.

One of the examples went to extraordinary lengths to include the character sequence `*/` within the example. That example has been replaced by an external snippet in a separate source file, which does not have any restriction on the use of `*/`. However, it does require that the file be excluded from standard compilation, and that is done by setting `EXCLUDES`, once for the "interim" compiler, and once again for the "product" compiler.    Going forward, a better solution might be to modify the `SetupJavaCompilation` macro to ignore all directories whose name is not a valid Java identifier (or, if easier, contains a `-`, such as `doc-files` or `snippet-files`.)

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Commit messages:
 - JDK-8272945: Use snippets in java.compiler documentation

Changes: https://git.openjdk.java.net/jdk/pull/6686/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=6686&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8272945
  Stats: 123 lines in 7 files changed: 51 ins; 25 del; 47 mod
  Patch: https://git.openjdk.java.net/jdk/pull/6686.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/6686/head:pull/6686

PR: https://git.openjdk.java.net/jdk/pull/6686