RFR: 8272554: Add help message about the named pipe support of UL.

[Xin Liu]

This patch adds a help message about the named pipe support of the Unified Logging to -Xlog:help and manpage.

For -Xlog:help, here is the new help message:

Available log outputs:
 stdout/stderr
 file=<filename>
  If the filename contains %p and/or %t, they will expand to the JVM's PID and startup timestamp, respectively. The filename can be a named pipe when log rotation is disabled.  


Here is the new manpage content.

 When using file=filename, specifying %p and/or %t in the file name expands to the JVM's PID and startup timestamp, respectively.  You can also configure text files to handle file rotation based on file size and a number of files to rotate.  For example, to rotate the log file every 10 MB and  keep  5
 files  in rotation, specify the options filesize=10M, filecount=5.  The target size of the files isn't guaranteed to be exact, it's just an approximate value.  Files are rotated by default with up to 5 rotated files of target size 20 MB, unless configured otherwise.  Specifying filecount=0 means that
 the log file shouldn't be rotated.  The filename can be a named pipe when log rotation is disabled.  There's a possibility of the pre-existing log file getting overwritten.

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Commit messages:
 - 8272554: Add help message about the named pipe support of UL.

Changes: https://git.openjdk.java.net/jdk/pull/5205/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=5205&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8272554
  Stats: 3 lines in 2 files changed: 2 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk/pull/5205.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/5205/head:pull/5205

PR: https://git.openjdk.java.net/jdk/pull/5205