XML docs for LINQ Join, LeftJoin, RightJoin (#111177)

* XML docs for LINQ Join, LeftJoin, RightJoin

See #110872 (comment)

* Remove nullability annotations in the docs

* Fix some incorrect xml code references

* Correct sample method names

* Correct summaries

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Co-authored-by: Jeff Handley <[email protected]>
Co-authored-by: Eirik Tsarpalis <[email protected]>

Author: 3 people