[FLINK-20288][docs] Correct documentation about savepoint self-contained

[Myasuka]

What is the purpose of the change

Correct documentation about savepoint self-contained

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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[Myasuka]

@klion26 , could you please take a view on this?

[klion26]

@Myasuka thanks for your contribution, maybe this is duplicated with FLINK-19381/PR ?

[StephanEwen]

I made a suggestion about how to phrase the section. Please let me know what you think.

[StephanEwen]

Can we change this to "...location on a distributed file-system or Object Store." ?

[StephanEwen]

What do you think about changing this to:

Savepoints can generally be moved by moving (or copying) the entire savepoint directory to a different location, and Flink will be able to restore from the moved savepoint.

<div class="alert alert-warning">An exception to that is if *entropy injection* is activated: In that case the savepoint directory will not contain all savepoint data files, because the injected path entropy spreads the files over many directories. Lacking a common savepoint root directory, the savepoints will contain absolute path references, which prevent moving the directory.</div>

<div class="alert alert-warning">Unlike savepoints, checkpoints cannot generally be moved to a different location, because checkpoints may include some absolute path references.</div>

If you use the `MemoryStateBackend`, metadata *and* savepoint state will be stored in the `_metadata` file, so don't be confused by the absence of additional data files.

I would drop the reference to a closed JIRA issue here, users should not feel like they need to look into the bug tracker to understand a feature.

[carp84]

@klion26 I think you're right, this PR duplicates #13488 . Please check out Stephan's comments and address them in #13488 as well, thanks.

@Myasuka @StephanEwen I'm closing this PR and let's further track it in #13488 , thanks.