Add notes to BULK_MANAGER.md to make removeBulkAction and addBulkAction more obvious

[asecondwill]

Description

I got muddled up yesterday trying to specify the classname to add and remove actions. Obvious once you know.

Here I am fumbling around on SO.
https://stackoverflow.com/questions/77781532/adding-and-removing-actions-from-silverstripe-bulkmanager/77786065#77786065

So some notes here to help others (me in a few months)

Manual testing steps

Issues

Pull request checklist

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[GuySartorelli]

Hi there,

Thank you for your contribution.
I notice that you didn't follow the pull request template - I've added that template back in to your pull request description. Please fill in all of the relevant information and follow the documented contribution guidelines.