-The argument TYPE (default `PRIMARY') says which selection,
-and DATA specifies the contents. DATA may be a string,
-a symbol, an integer (or a cons of two integers or list of two integers).
+The argument TYPE (nil means `PRIMARY') says which selection, and
+DATA specifies the contents. TYPE must be a symbol. \(It can also
+be a string, which stands for the symbol with that name, but this
+is considered obsolete.) DATA may be a string, a symbol, an
+integer (or a cons of two integers or list of two integers).