[ES|QL] Provide signature help

[dej611]

Describe the feature:

A nice feature in Lens formula is the signature help which highlights the current definition of the used function, together with some doc/example, and positional information about arguments:

I've made a quick PoC with ES|QL and the feature seems feasible:

Describe a specific use case for the feature:

It would be nice to implement this feature, and also leverage the signature information for more (i.e. validation or better autocomplete).

[elasticmachine]

Pinging @elastic/kibana-visualizations (Team:Visualizations)

[ninoslavmiskovic]

@dej611 nice 👍 Where does it pull the content from ? We have been talking about unifying the documentation, so in this example does it pull the content from FROM documentation?

[dej611]

Currently there are two source of documentation in the editor: one for the popover panel and another for the suggestion system (hovering an entry will show a right arrow that the user can click to show some short documentation as well).
Ideally both should be merged together into a single source and each medium should provide the portion of information required only.
For this feature we could hook into this system and reuse the same source.

[elasticmachine]

Pinging @elastic/kibana-esql (Team:ESQL)

[stratoula]

Closing this in favor of #180528