RFR: 8310405: Linker.Option.firstVariadicArg should specify which index values are valid

[Jorn Vernee]

Improve the specification of `Linker.Option.firstVariadicArg`, by specifying more clearly which index values are valid.

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Commit messages:
 - improve firstVariadicArg doc

Changes: https://git.openjdk.org/jdk/pull/14565/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=14565&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8310405
  Stats: 10 lines in 1 file changed: 10 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/14565.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/14565/head:pull/14565

PR: https://git.openjdk.org/jdk/pull/14565