Better docs for Match.Optional.

docs/client/api.html

@@ -2344,14 +2344,13 @@ The following patterns can be used as pattern arguments to `check` and `Match.te
 Matches any value.
 {{/dtdd}}
 
-{{#dtdd "<code>Match.Integer</code>"}}
-Matches a signed 32-bit integer.
-{{/dtdd}}
-
 {{#dtdd "<code>String</code>, <code>Number</code>, <code>Boolean</code>, <code>undefined</code>, <code>null</code>"}}
 Matches a primitive of the given type.
 {{/dtdd}}
 
+{{#dtdd "<code>Match.Integer</code>"}}
+Matches a signed 32-bit integer. Doesn't match `Infinity`, `-Infinity`, or `NaN`.
+{{/dtdd}}
 
 {{#dtdd "<code>[<em>pattern</em>]</code>"}}
 A one-element array matches an array of elements, each of which match
@@ -2376,8 +2375,19 @@ Matches any plain Object with any keys; equivalent to
 `Match.ObjectIncluding({})`.
 {{/dtdd}}
 
-{{#dtdd "<code>Match.Optional(<em>pattern</em>)</code>"}}
-Matches either `undefined` on the top level or absence in an object or something that matches *pattern*.
+{{#dtdd "<code>Match.Optional(<em>pattern</em>)</code>"}} Matches either
+`undefined` or something that matches pattern. If used in an object this matches
+only if the key is not set as opposed to the value being set to `undefined`.
+
+    // In an object
+    var pat = { name: Match.Optional(String) };
+    check({ name: "something" }, pat) // OK
+    check({}, pat) // OK
+    check({ name: undefined }, pat) // Throws an exception
+
+    // Outside an object
+    check(undefined, Match.Optional(String)); // OK
+
 {{/dtdd}}
 
 {{#dtdd "<code>Match.OneOf(<em>pattern1</em>, <em>pattern2</em>, ...)</code>"}}