[jdk16] RFR: JDK-8253996: Javac error on jdk16 build 18: invalid flag: -Xdoclint:-missing [v2]

[Jonathan Gibbons]

> This PR improves the diagnostic when javac is run in an image that does not provide a service provider for DocLint.
> 
> Instead of reporting messages about unsupported options, the following warning will be generated:
> 
> `
> Warning: No service provider for doclint is available
> `
> 
> In addition, as a minor cleanup, in the `jdk.javadoc` module, javadoc b bypasses the service loader to access `doclint` directly.

Jonathan Gibbons has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains three additional commits since the last revision:

 - Fix comment
 - Merge remote-tracking branch 'upstream/master' into doclint.missing
 - JDK-8253996: Javac error on jdk16 build 18: invalid flag: -Xdoclint:-missing

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

Changes:
  - all: https://git.openjdk.java.net/jdk16/pull/60/files
  - new: https://git.openjdk.java.net/jdk16/pull/60/files/f9aacd5c..595d31d1

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk16&pr=60&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jdk16&pr=60&range=00-01

  Stats: 3646 lines in 117 files changed: 1146 ins; 2369 del; 131 mod
  Patch: https://git.openjdk.java.net/jdk16/pull/60.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk16 pull/60/head:pull/60

PR: https://git.openjdk.java.net/jdk16/pull/60