improvements to set reference

Author: schmalliso

source/reference/operator/update/set.txt

@@ -9,7 +9,7 @@ Definition
 
 .. update:: $set
 
-   The :update:`$set` operator replaces the value of a field to the
+   The :update:`$set` operator replaces the value of a field with the
    specified value.
 
    The :update:`$set` operator expression has the following form:
@@ -23,11 +23,16 @@ Definition
 Behavior
 --------
 
-If the field does not exist, :update:`$set` will add the field with the
-specified value. If you specify a dotted path for a non-existent field,
+If the field does not exist, :update:`$set` will add a new field with the
+specified value, provided that the new field does not violate a type
+constraint.
+If you specify a dotted path for a non-existent field,
 :update:`$set` will also create the embedded documents *as needed* to
 fulfill the dotted path to the field.
 
+If you specify multiple field-value pairs, :update:`$set` will update
+or create each field.
+
 Examples
 --------