RFR: 8261431: SA: Add comments about load address of executable

[Yasumasa Suenaga]

I removed the comment about load address of executable in [JDK-8248876](https://bugs.openjdk.java.net/browse/JDK-8248876) (#2366), but it contained useful information for maintenance.

So I re-add them, and add comment for the change in JDK-8248876. Please see https://github.com/openjdk/jdk/pull/2366#issuecomment-775362982

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Commit messages:
 - 8261431: SA: Add comments about load address of executable

Changes: https://git.openjdk.java.net/jdk/pull/2478/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=2478&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8261431
  Stats: 4 lines in 1 file changed: 4 ins; 0 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk/pull/2478.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/2478/head:pull/2478

PR: https://git.openjdk.java.net/jdk/pull/2478