add retrieve description

[YuliyaPoznyak]

No description provided.

[georged]

@YuliyaPoznyak we need to add these 2 functions to the main documentation and link to that KB. We should explain (it's ok in both places, main docs and KB) that these functions provide direct low-level access to Web API methods (link to Microsoft docs). This is an alternative to fetchxml that is more flexible and has better performance but requires more in-depth understanding of Web API.

I expanded the title to make clear what we're doing here.

Also, please double-check the tags (if Twig is the correct one, specifically)

[georged]

@YuliyaPoznyak for twig examples in the future please use twig as the code language not php