RFR: 8314684: Add overview docs to loaderConstraints.cpp

[Ioi Lam]

I added some documentation to make it easier to understand loaderConstraints.cpp. Please review and check if my description is actually correct. Thanks!

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Commit messages:
 - 8314684: Add overview docs to loaderConstraints.cpp

Changes: https://git.openjdk.org/jdk/pull/15372/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=15372&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8314684
  Stats: 41 lines in 1 file changed: 40 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/15372.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/15372/head:pull/15372

PR: https://git.openjdk.org/jdk/pull/15372