RFR: 8290353: Clarify the streams returned by ModuleReader::list

[Ryan Ernst]

This commit adds additional clarification to the javadocs of
ModuleReader::list about needing to close the stream. The new wording is
similar to that used in Files::find and other similar methods.

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Commit messages:
 - 8290353: Clarify the streams returned by ModuleReader::list

Changes: https://git.openjdk.org/jdk/pull/9538/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=9538&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8290353
  Stats: 8 lines in 1 file changed: 8 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/9538.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/9538/head:pull/9538

PR: https://git.openjdk.org/jdk/pull/9538