RFR: 7904086: Fix docs that JUnit tests need no `@build` for `@library` contents

[Volkan Yazici]

If a library directory is used by a directory of TestNG **or JUnit** tests, all the classes in a library directory will be automatically compiled by means of implicit `@build` tags. Amend the docs accordingly.

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Commit messages:
 - Fix docs that JUnit tests need no `@build` for `@library` contents

Changes: https://git.openjdk.org/jtreg/pull/292/files
  Webrev: https://webrevs.openjdk.org/?repo=jtreg&pr=292&range=00
  Issue: https://bugs.openjdk.org/browse/CODETOOLS-7904086
  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jtreg/pull/292.diff
  Fetch: git fetch https://git.openjdk.org/jtreg.git pull/292/head:pull/292

PR: https://git.openjdk.org/jtreg/pull/292