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

[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.

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

Commit messages:
 - JDK-8253996: Javac error on jdk16 build 18: invalid flag: -Xdoclint:-missing

Changes: https://git.openjdk.java.net/jdk16/pull/60/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk16&pr=60&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8253996
  Stats: 96 lines in 6 files changed: 88 ins; 1 del; 7 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