DOCS-5169: Fix missing brace in $push, rephrase intro in $addToSet

Author: i80and

source/reference/operator/update/addToSet.txt

@@ -9,9 +9,8 @@ Definition
 
 .. update:: $addToSet
 
-   The :update:`$addToSet` operator adds a value to an array only *if* the
-   value is *not* already in the array. If the value *is* in the
-   array, :update:`$addToSet` does not modify the
+   The :update:`$addToSet` operator adds a value to an array unless the value
+   is already present, in which case :update:`$addToSet` does nothing to that
    array.
 
    The :update:`$addToSet` operator has the form:

source/reference/operator/update/push.txt

@@ -15,7 +15,7 @@ Definition
 
    .. code-block:: javascript
 
-      { $push: { <field1>: <value1>, ... }
+      { $push: { <field1>: <value1>, ... } }
 
    .. include:: /includes/use-dot-notation.rst