Updated RELATEDTABLE documentation with content from Learning Path

[equerystrian]

I initially posted this in the forums but was recommended to head over here. 

I've been working through the DAX learning path on Learn & found a tip on RELATEDTABLE that was so wonderfully clear I wanted to share it with the world:

"Notice that the formula uses the RELATED function. Remember, row context does not extend beyond the table. If your formula needs to reference columns in other tables, and there are model relationships between the tables, use the RELATED function for the one-side, or the RELATEDTABLE function for the many-side."

I immediately went to the RELATEDTABLE docs page to see if this gem existed there as well. Sadly, this page's functional documentation is very sparse. I went to check the contributors but the Github link is broken. 

If there are any powers who can lift that wonderful Learn content and get it to the DAX documentation page it would be well worth your time!

[v-chuncz-msft]

@equerystrian 

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