doc: Added explanation of preceding/following controllers (backport #2192)

[mergify]

Resolves #2189 .

This PR adds explanation of what preceding/following controller means.

I also broke the rule and attached an irrelevant fix of grammar in the paragraph directly preceding the paragraph I added. I hope it's not a big no no :)

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This is an automatic backport of pull request #2192 done by Mergify.

[mergify]

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	both modified:   controller_manager/doc/controller_chaining.rst

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