RFR: JDK-8274639: Provide a way to disable warnings for cross-modular links

[Hannes Wallnöfer]

This is a simple change to add a new `--link-modularity-mismatch (warn|info)` command line option to allow messages about wrong modularity of externally linked documentation bundles to be issued as warnings or notices. 

CSR: https://bugs.openjdk.java.net/browse/JDK-8274969

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Commit messages:
 - JDK-8274639: Provide a way to disable warnings for cross-modular links
 - JDK-8274639: Avoid warnings when creating cross-modular links

Changes: https://git.openjdk.java.net/jdk/pull/5900/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=5900&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8274639
  Stats: 122 lines in 4 files changed: 99 ins; 6 del; 17 mod
  Patch: https://git.openjdk.java.net/jdk/pull/5900.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/5900/head:pull/5900

PR: https://git.openjdk.java.net/jdk/pull/5900