RFR: 8274605: Fix predicate guarantees on returned values in (Doc)SourcePositions

[Pavel Rappo]

Please review this documentation change. The result might look verbose, but at least it is no longer confusing.

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Commit messages:
 - Initial commit

Changes: https://git.openjdk.java.net/jdk/pull/5781/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=5781&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8274605
  Stats: 20 lines in 2 files changed: 0 ins; 0 del; 20 mod
  Patch: https://git.openjdk.java.net/jdk/pull/5781.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/5781/head:pull/5781

PR: https://git.openjdk.java.net/jdk/pull/5781