document 'on conflict' in query language guide

documentation/src/main/asciidoc/querylanguage/Concepts.adoc

@@ -427,6 +427,24 @@ It's not available for entities whose id generator is implemented in Java, nor f
 The same two options are available for a `@Version` attribute.
 When no version is explicitly specified, the version for a new entity instance is used.
 
+The `on conflict` clause lets us specify what action should be taken when the database already contains the record we're attempting to insert.
+
+[source, antlrv4]
+----
+include::{extrasdir}/on_conflict_bnf.txt[]
+----
+
+Note that the `on constraint` variant accepting the name of a unique constraint only works on certain databases, or when just a single row is being inserted.
+
+[[onconflict-example]]
+[source, hql]
+----
+insert Person (ssn, name, phone)
+    values ('116-76-1234', 'Jane Doe', '404 888 4319')
+    on conflict (ssn) set phone = excluded.phone
+----
+
+
 Like `update` and `delete` statements, an `insert` statement must be executed by calling `Query.executeUpdate()`.
 
 Now it's time to look at something _much_ more complicated.

documentation/src/main/asciidoc/querylanguage/extras/on_conflict_bnf.txt

@@ -0,0 +1,10 @@
+conflictClause
+	: ON CONFLICT conflictTarget? "DO" conflictAction
+
+conflictTarget
+	: ON CONSTRAINT identifier
+	| "(" simplePath ("," simplePath)* ")"
+
+conflictAction
+	: "NOTHING"
+	| "UPDATE" setClause whereClause?

documentation/src/main/asciidoc/querylanguage/extras/statement_insert_bnf.txt

@@ -1,5 +1,7 @@
 insertStatement
-    : "INSERT" "INTO"? targetEntity targetFields (queryExpression | valuesList)
+    : "INSERT" "INTO"? targetEntity targetFields
+      (queryExpression | valuesList)
+      conflictClause?
 
 targetEntity
 	: entityName variable?