RFR: 6704: More information when opening an LZ4 compressed recording

[Marcus Hirt]

Since LZ4 works great when compressing flight recordings, we should at least warn if LZ4 compressed recordings are encountered.

(We will eventually support it in JMC 8, but this warning should be back ported to JMC 7.)

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

Commits:
 - f4c001f0: Better javadoc
 - d1a4cc3f: 6704: Warning when opening LZ4 compressed recording

Changes: https://git.openjdk.java.net/jmc/pull/62/files
 Webrev: https://webrevs.openjdk.java.net/jmc/62/webrev.00
  Issue: https://bugs.openjdk.java.net/browse/JMC-6704
  Stats: 31 lines in 4 files changed: 31 ins; 0 del; 0 mod
  Patch: https://git.openjdk.java.net/jmc/pull/62.diff
  Fetch: git fetch https://git.openjdk.java.net/jmc pull/62/head:pull/62

PR: https://git.openjdk.java.net/jmc/pull/62