Emphasize important difference between Kuery and Lucene query syntax (e…

…lastic#13592)

Fixes elastic#13563

Brings additional attention to the fact that Kuery uses implicit ANDs whereas Lucene query syntax uses implicit ORs, which could confuse users if they're not aware of the difference.

docs/discover/kuery.asciidoc

@@ -17,7 +17,8 @@ Multiple search terms are separated by whitespace.
 
 `response:200 extension:php` will match documents where response matches 200 and extension matches php.
 
-All terms must match by default. The language supports boolean logic with and/or operators. The above query is equivalent to `response:200 and extension:php`.
+*All terms must match by default*. The language supports boolean logic with and/or operators. The above query is equivalent to `response:200 and extension:php`.
+This is a departure from the Lucene query syntax where all terms are optional by default.
 
 We can make terms optional by using `or`.