Fix list of macro roles that need "names:" argument (#375)

Fixes: #374
Reviewed-by: Holly Borla <[email protected]>

Author: amartini51

TSPL.docc/ReferenceManual/Attributes.md

@@ -97,8 +97,12 @@ indicates the macro's role:
   on an extension, a type alias, or a type that's nested inside a function,
   or use an extension macro to add an extension that has a peer macro.
 
-The peer, member, and accessor macro roles require a `names:` argument,
+The peer and member macro roles require a `names:` argument,
 listing the names of the symbols that the macro generates.
+The accessor macro role requires a `names:` argument if the
+macro generates a `willSet` or `didSet` property observer. An
+accessor macro that generates property observers can't add
+other accessors, because observers only apply to stored properties.
 The extension macro role also requires a `names:` argument
 if the macro adds declarations inside the extension.
 When a macro declaration includes the `names:` argument,