docs(mongodb): document new functionalities

[kamilmysliwiec]

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What kind of change does this PR introduce?

[ ] Bugfix
[ ] Feature
[ ] Code style update (formatting, local variables)
[ ] Refactoring (no functional changes, no api changes)
[ ] Build related changes
[ ] Other... Please describe:

What is the current behavior?

Issue Number: N/A

What is the new behavior?

Does this PR introduce a breaking change?

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[ ] No

Other information

[johnbiundo]

Suggestion: add the following:

As with pre() and post() hooks, use the useFactory technique to access a schema in order to invoke the plugin() method.

[johnbiundo]

@kamilmysliwiec I made several comments. I'm unfortunately not in a position for the next week to actually test this use case, so please consider my suggestions carefully to make sure I do not inadvertently change the meaning. My intention is to make some minor changes for clarity only.

[kamilmysliwiec]

Look good @johnbiundo. Thank you!!