RFR: 6426678: (spec) File.createTempFile(prefix, suffix, dir) needs clarification for illegal symbols in suffix

[Brian Burkhalter]

Add some verbiage indicating that an `IOException` will be thrown if a file with a name generated from the supplied prefix and suffix according to the described algorithm cannot be generated by the underlying system, whether that be due to the presence of one of more characters not supported by the underlying system or for some other reason.

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Commit messages:
 - 6426678: (spec) File.createTempFile(prefix, suffix, dir) needs clarification for illegal symbols in suffix

Changes: https://git.openjdk.org/jdk/pull/20435/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=20435&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-6426678
  Stats: 6 lines in 1 file changed: 5 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/20435.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/20435/head:pull/20435

PR: https://git.openjdk.org/jdk/pull/20435