Add Python API docs for RebinParamsValidator

[soininen]

Add the missing .rst file and docstring for RebinParamsValidator.

Report to: [nobody].

To test:

Build the all and docs-html targets and check that the documentation for mantid.kernel.RebinParamsValidator appears in the correct place.

There is no associated issue.

This does not require release notes because it is an insignificant addition to the API docs.

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Reviewer

Please comment on the following (full description):

Code Review

• Is the code of an acceptable quality?
• Does the code conform to the coding standards?
• Are the unit tests small and test the class in isolation?
• If there are changes in the release notes then do they describe the changes appropriately?

Functional Tests

• Do changes function as described? Add comments below that describe the tests performed?
• Do the changes handle unexpected situations, e.g. bad input?
• Has the relevant (user and developer) documentation been added/updated?

Does everything look good? Mark the review as Approve. A member of @mantidproject/gatekeepers will take care of it.