[Documentation] Common extended selectors doc

[Telroshan]

As discussed in #1522 and #1478 with @alexpetros

Several attributes use the extended selectors set, such as hx-target, hx-include, hx-indicator, and hx-trigger's from clause.

As of today though, there's no common documentation for that selectors syntax, it's up to each attribute's doc to give details about it or not.
As such, docs like hx-include don't mention them at all, while a doc like hx-indicator only mentions closest.

All of those attributes, even hx-trigger's from clause, result in a call to querySelectorAllExt.

It would be nice to provide a separate documentation for that extended CSS selectors syntax (especially as we'll probably add more features to it in the future, such as #1597), and link to that doc in every attribute using a selector.

[danielo515]

Thanks I found this issue. I am adding loading indicators and being limited to closest was making things very hard

[Telroshan]

Depending on the outcome of #1962, HX-Retarget may also need that common extended selectors documentation link