[FLINK-27327][docs] Add description about changing max parallelism ex…

[masteryhx]

What is the purpose of the change

This pull request adds describtion about changing max parallelism explicitly leads to state incompatibility.

Brief change log

• Added "changing maxParallelism/pipline.max-parallelism explicitly when recovery from original job will lead to state incompatibility" into parallel.md and the description of pipline.max-parallelism.

Verifying this change

This change is a trivial work of document 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, 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? no

[flinkbot]

CI report:

• 6d0c203 Azure: SUCCESS

Bot commands

The @flinkbot bot supports the following commands:

• @flinkbot run azure re-run the last Azure build

[Myasuka]

Thanks for creating this PR. LGTM.