RFR: 8314684: Add overview docs to loaderConstraints.cpp [v3]

[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!

Ioi Lam has updated the pull request incrementally with two additional commits since the last revision:

 - added references to source code
 - added info about ConstraintSet

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Changes:
  - all: https://git.openjdk.org/jdk/pull/15372/files
  - new: https://git.openjdk.org/jdk/pull/15372/files/557eafa3..786971e9

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=15372&range=02
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=15372&range=01-02

  Stats: 6 lines in 1 file changed: 3 ins; 0 del; 3 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