RFR: CODETOOLS-7903024: Improve docs and impl for jtreg -show option

[Jonathan Gibbons]

This is a PR to ...

* improve the`-show` option to allow a section name to be specified as well as a stream name
* improve the command-line help for `-show`, to emphasize that it displays the unencoded stream contents
* improve the FAQ with related new questions and links

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Commit messages:
 - CODETOOLS-7903024: Improve docs and impl for jtreg -show option

Changes: https://git.openjdk.java.net/jtreg/pull/26/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jtreg&pr=26&range=00
  Issue: https://bugs.openjdk.java.net/browse/CODETOOLS-7903024
  Stats: 74 lines in 4 files changed: 58 ins; 0 del; 16 mod
  Patch: https://git.openjdk.java.net/jtreg/pull/26.diff
  Fetch: git fetch https://git.openjdk.java.net/jtreg pull/26/head:pull/26

PR: https://git.openjdk.java.net/jtreg/pull/26