RFR: 8371133: Clarify the purpose of "src/jdk.compiler/share/classes/com/sun/tools/javac/resources/ct.properties"

[Volkan Yazici]

Document usage and purpose of `ct.properties`.

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Commit messages:
 - Document `ct.properties`

Changes: https://git.openjdk.org/jdk/pull/28123/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=28123&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8371133
  Stats: 9 lines in 1 file changed: 8 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/28123.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/28123/head:pull/28123

PR: https://git.openjdk.org/jdk/pull/28123