ESQL: document boolean operators' behavior on NULLs

[alex-spies]

This is coming up every now and then, and we'll get bug reports where a WHERE command behaves unexpectedly in case of null values in the fields. Most recently, this was raised in #129125.

I checked our docs and couldn't find an explanation of the ternary logic (true, false, null) in our docs. The crux is that

• when an expression is null, combining with NOT, AND and OR will still result in null (for OR: unless the other operand is true, for AND: unless the other operand is false)
  and for a good reason: null means "I don't know the value", and thus the output of null AND true can only be null.

I think it'd be nice to structure our docs in a way that boolean operators are grouped together and give each of them a 3x3 (resp. 3x1) table of how they evaluate in case of true, false and null in each of their operands. Additionally, giving an example would be great - like the example from the linked issue:

• WHERE process.name IS NULL AND NOT process.name == "svchost.exe" (this evaluates to WHERE null, which filters out all the rows - but this is expected, see my comment for an explanation)

[elasticsearchmachine]

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

[elasticsearchmachine]

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

[alex-spies]

I just realized we already had a similar issue for this, #120075. I'll keep this around due to being less focused on aggs.