RFR: 4750574: (se spec) Selector spec should clarify calculation of select return value

[Brian Burkhalter]

Improve the description of the return value of each of the `Selector.select()` methods which do not accept an action parameter.

-------------

Commit messages:
 - 4750574: (se spec) Selector spec should clarify calculation of select return value

Changes: https://git.openjdk.java.net/jdk/pull/7220/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=7220&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-4750574
  Stats: 9 lines in 1 file changed: 3 ins; 0 del; 6 mod
  Patch: https://git.openjdk.java.net/jdk/pull/7220.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/7220/head:pull/7220

PR: https://git.openjdk.java.net/jdk/pull/7220