[FLINK-20371][docs] Add docs for outer interval joins

[twalthr]

What is the purpose of the change

Add docs for outer interval joins

Brief change log

Add docs for outer interval joins

Verifying this change

This change is a trivial rework / code cleanup without any test coverage.

Does this pull request potentially affect one of the following parts:

• Dependencies (does it add or upgrade a dependency): no
• The public API, i.e., is any changed class annotated with @Public(Evolving): no
• The serializers: no
• The runtime per-record code paths (performance sensitive): no
• Anything that affects deployment or recovery: JobManager (and its components), Checkpointing, Kubernetes/Yarn/Mesos, ZooKeeper: no
• The S3 file system connector: no

Documentation

• Does this pull request introduce a new feature? no
• If yes, how is the feature documented? not applicable

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Last check on commit 33d6610 (Thu Nov 26 09:04:12 UTC 2020)

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[wuchong]

LGTM.

[sjwiesman]

can we merge this? I'm working on generally improving the sql join page.

[twalthr]

@sjwiesman sure, go ahead

[sjwiesman]

merging ...

master: e8d6b29

release 1.12 28f3aba