RFR: 8266901: Clarify the method description of Duration.toDaysPart()

[Naoto Sato]

Please review this doc clarification fix to `toDaysPart()` method. CSR will also be filed accordingly.

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Commit messages:
 - 8266901: Clarify the method description of Duration.toDaysPart()

Changes: https://git.openjdk.java.net/jdk/pull/4542/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=4542&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8266901
  Stats: 3 lines in 1 file changed: 2 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk/pull/4542.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/4542/head:pull/4542

PR: https://git.openjdk.java.net/jdk/pull/4542