Remove wording around macro name omission (#393)

* Update wording around anonymous macros to match existing conformance tests and implementations

* Adjust wording based on PR feedback

Author: nirosys

_books/ion-1-1/src/macros/defining_macros.md

@@ -7,11 +7,11 @@ A macro is defined using a `macro` clause within a [module](../modules.md)'s [`m
 (macro name signature template)
 ```
 
-| Argument                                        | Description                                                                                               |
-|-------------------------------------------------|-----------------------------------------------------------------------------------------------------------|
-| [`name`](#macro-names)                          | A unique name assigned to the macro. When constructing an anonymous macro, this argument is omitted.      |
-| [`signature`](#macro-signatures)                | An s-expression enumerating the parameters this macro accepts.                                            |
-| [`template`](#template-definition-language-tdl) | A template definition language (TDL) expression that can be evaluated to produce zero or more Ion values. |
+| Argument                                        | Description                                                                                                                    |
+|-------------------------------------------------|--------------------------------------------------------------------------------------------------------------------------------|
+| [`name`](#macro-names)                          | A unique name assigned to the macro. When constructing an anonymous macro `null` is used in the place of a unique name.        |
+| [`signature`](#macro-signatures)                | An s-expression enumerating the parameters this macro accepts.                                                                 |
+| [`template`](#template-definition-language-tdl) | A template definition language (TDL) expression that can be evaluated to produce zero or more Ion values.                      |
 
 ### Example macro clause
 ```ion
@@ -32,7 +32,7 @@ A macro is defined using a `macro` clause within a [module](../modules.md)'s [`m
 Syntactically, macro names are [identifiers](../text/symbol-tokens.md). Each macro name in a macro table must be unique.
 
 In some circumstances, it may not make sense to name a macro. (For example, when the macro is generated automatically.)
-In such cases, authors may omit the macro name to indicate that the macro does not have a name.
+In such cases, authors must use `null` to indicate that the macro does not have a name.
 Anonymous macros can only be referenced by their address in the macro table.
 
 ## Macro parameters

_books/ion-1-1/src/modules/defining_modules.md

@@ -119,7 +119,7 @@ Most commonly, a macro table entry is a definition of a new macro expansion func
 ```
 (See the [_Defining macros_](../macros/defining_macros.md) for details.)
 
-When no name is given, this defines an anonymous macro that can be referenced by its numeric
+When the value `null` is given for the macro name, this defines an anonymous macro that can be referenced by its numeric
 address (that is, its index in the enclosing macro table).
 Inside the defining module, that uses a local reference like `12`.