Document the need to use double embedding when doing OR filtering across embeds

[laurenceisla]

For example, when a user wants to get info from table with an embedded rel where table.col=val OR where rel.rel_col=val, they do this:

/table?select=*,rel(*)&rel.rel_col=eq.val&or=(base_col.eq.val,rel.not.is.null)

But this will return {"rel": null} when the table.col=val is fulfilled and when rel.rel_col=val is not. This is intended, but the user usually wants to show the values of rel even if it doesn't satisfy rel.rel_col=val, which is solved by embedding twice:

/table?select=*,r:rel(),rel(*)&r.rel_col=eq.val&or=(base_col.eq.val,r.not.is.null)

We may need to add an explanation to the OR across embeds section since it has been asked quite often recently, e.g.:

• OR with a mix of root and embedded resources #3333 (reply in thread)
• Filtering With Or Logical Operator on Embedded Resources Inconsistent Behavior #3349