RFR: 8255797: ciReplay: improve documentation of replay file syntax in parser

[Roberto Castañeda Lozano]

Complete and disambiguate the informal specification of the replay file syntax
given in the ciReplay class implementation.

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Commit messages:
 - 8255797: ciReplay: improve documentation of replay file syntax in parser

Changes: https://git.openjdk.java.net/jdk/pull/1033/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=1033&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8255797
  Stats: 10 lines in 1 file changed: 3 ins; 2 del; 5 mod
  Patch: https://git.openjdk.java.net/jdk/pull/1033.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/1033/head:pull/1033

PR: https://git.openjdk.java.net/jdk/pull/1033