ES|QL: review, double check and add missing bits to functions documentation

[astefan]

Description

I have identified multiple points that can be improved with said documentation so far (there may be others and the list is not exhaustive):

• values function is missing a crucial aspect: it's deduplicating the values it finds
• mv_slice doesn't mention that it uses 0-based indexing
• there are multiple mentions like the one for mv_zip that specify the parameter as being "multivalue expression". This implies some sort of soft requirement, but in reality the parameter can be null, single value, multi value.
• some math functions could use a reference link to the description or formula for them. For example, mv_median could use the link from wikipedia. acos is the better example.
• date_trunc mentions "timespan literal syntax" and "time_duration" / "date_period" but there is no link for any of these, listing what they are and the possible values ES|QL: document date_period and time_duration data types #112592

[elasticsearchmachine]

Pinging @elastic/es-docs (Team:Docs)

[elasticsearchmachine]

Pinging @elastic/es-analytical-engine (Team:Analytics)

[alex-spies]

Let's also address multivalue functions, which can confuse users when there's a corresponding aggregate function. We could add a general disclaimer to the multivalue functions section in our docs, and also a short sentence to functions like mv_max on how they're different from their aggregate functions e.g. max.