SQL: [docs] Add documentation for COALESCE (#35740)

Follows: #35253

docs/reference/sql/functions/conditional.asciidoc

@@ -0,0 +1,46 @@
+[role="xpack"]
+[testenv="basic"]
+[[sql-functions-conditional]]
+=== Conditional Functions
+
+Functions that return one of their arguments by evaluating in an if-else manner.
+
+[[sql-functions-conditional-coalesce]]
+==== `COALESCE`
+
+.Synopsis
+[source, sql]
+----
+COALESCE ( expression<1>, expression<2>, ... )
+----
+
+*Input*:
+
+<1> 1st expression
+
+<2> 2nd expression
+
+...
+
+**N**th expression
+
+COALESCE can take an arbitrary number of arguments.
+
+*Output*: one of the expressions or `null`
+
+.Description
+
+Returns the first of its arguments that is not null.
+If all arguments are null, then it returns `null`.
+
+
+
+["source","sql",subs="attributes,callouts,macros"]
+----
+include-tagged::{sql-specs}/docs.csv-spec[coalesceReturnNonNull]
+----
+
+["source","sql",subs="attributes,callouts,macros"]
+----
+include-tagged::{sql-specs}/docs.csv-spec[coalesceReturnNull]
+----