RFR: JDK-8278318: Create {@index} entries for key LangTools terms

[Jonathan Gibbons]

Please review a small update to the `jdk.compiler` and `java.compiler` module declarations, to index the names of some key compiler-related APIs.

There are no API changes, so no CSR is required.

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Commit messages:
 - JDK-8278318: Create {@index} entries for key LangTools terms

Changes: https://git.openjdk.java.net/jdk/pull/6729/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=6729&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8278318
  Stats: 7 lines in 2 files changed: 6 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk/pull/6729.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/6729/head:pull/6729

PR: https://git.openjdk.java.net/jdk/pull/6729