Description

Page URL: https://docs.qgis.org/3.34/en/docs/user_manual/processing_algs/qgis/vectorgeneral.html#qgisjoinattributesbylocation

Predicate Contain is described as follows:
"Contain
Returns 1 (true) if and only if no points of b lie in the exterior of a, and at least one point of the interior of b lies in the interior of a. In the picture, no circle is returned, but the rectangle would be if you would look for it the other way around, as it contains circle 1 completely. This is the opposite of are within."
however it seems to do what a predicate "Are within" does.
Either replace descriptions or move the functions to their proper descriptions.

Furtheremore, to make the description clearer replace "geometry a" with "geometry from layer 1" or alike, and "geometry b" with "geometry from layer 2", where 1 referes to input vector layer and 2 referes to the join layer. This should be made throutghout the text.

Check 27.1.18.20. Join attributes by location (summary) for the same issues.