RFR: JDK-8314115: DatagramSocket.receive may hang if underlying UDP RCVBUF of OS is too small

[Thomas Obermeier]

docu update to explain that receive can hang because of bad configuration

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

Commit messages:
 - 8314115: docu update DatagramSocket.receive() potential hang
 - 8314115: trailing whitespaces
 - 8314115: docu update DatagramChannel.receive() potential hang

Changes: https://git.openjdk.org/jdk/pull/15401/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=15401&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8314115
  Stats: 10 lines in 2 files changed: 10 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/15401.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/15401/head:pull/15401

PR: https://git.openjdk.org/jdk/pull/15401