Clarify the relationship between declaration patterns and is-type

Fixes #1661

Author: jskeet

standard/patterns.md

@@ -94,9 +94,11 @@ When recognising a *simple_designation* if both the *discard_designation* and *s
 
 > *Note*: ANTLR makes the specified choice automatically due to the ordering of the alternatives of *simple_designation*. *end note*
 
-The runtime type of the value is tested against the *type* in the pattern using the same rules specified in the is-type operator ([§12.14.12.1](expressions.md#1214121-the-is-type-operator)). If the test succeeds, the pattern *matches* that value. It is a compile-time error if the *type* is a nullable value type ([§8.3.12](types.md#8312-nullable-value-types)) or a nullable reference type ([§8.9.3](types.md#893-nullable-reference-types)). This pattern form never matches a `null` value.
+It is a compile-time error if the *type* is a nullable value type ([§8.3.12](types.md#8312-nullable-value-types)) or a nullable reference type ([§8.9.3](types.md#893-nullable-reference-types)).
+
+The runtime type of the value is tested against the *type* in the pattern using the same rules specified in the is-type operator ([§12.14.12.1](expressions.md#1214121-the-is-type-operator)). If the test succeeds, the pattern *matches* that value.
 
-> *Note*: The is-type expression `e is T` and the declaration pattern `e is T _` are equivalent when `T` is not a nullable type. *end note*
+> *Note*: The is-type expression `e is T` and the declaration pattern `e is T _` are equivalent when both are valid. *end note*
 
 Given a pattern input value ([§11.1](patterns.md#111-general)) *e*, if the *simple_designation* is a *discard_designation*, denoting a discard ([§9.2.9.2](variables.md#9292-discards)), the value of *e* is not bound to anything. Otherwise, if the *simple_designation* is a *single_variable_designation*, a local variable ([§9.2.9](variables.md#929-local-variables)) of the given type named by the given identifier is introduced. That local variable is assigned the value of the pattern input value when the pattern *matches* the value.