Update attributes template per review changes

In reviewing and revising the work to move all attributes to the new
template, some patterns have emerged in editing.  Let's capture these
in the template.

Author: traviscross

docs/attribute-template.md

@@ -15,7 +15,8 @@ The *`example` [attribute][attributes]* ...give a high level description.
 
 > [!EXAMPLE]
 > ```rust
-> // This should be a very basic example showing the attribute used in some way.
+> // This should be a very basic example showing the attribute
+> // used in some way.
 > #[example]
 > fn some_meaningful_name() {}
 > ```
@@ -25,19 +26,70 @@ Describe the accepted syntax of this attribute. You can either explain that it u
 
 ----
 
-The `example` attribute uses the [MetaWord] syntax and thus does not take any inputs.
+The `example` attribute uses the [MetaWord] syntax.
 
 ----
 
 The `example` attribute uses the [MetaListPaths] syntax to specify a list of ...
 
 ----
 
+The `example` attribute uses the [MetaWord] and [MetaNameValueStr] syntaxes.
+
+----
+
+The `example` attribute uses the [MetaWord], [MetaListPaths], and [MetaNameValueStr] syntaxes.
+
+----
+
+The `example` attribute uses the [MetaNameValueStr] syntax. Accepted values are `"X"` and `"Y"`.
+
+----
+
+The `example` attribute uses the [MetaNameValueStr] syntax. The value in the string must be ...
+
+----
+
+The `example` attribute has these forms:
+
+- [MetaWord]
+  > [!EXAMPLE]
+  > ```rust
+  > #[example]
+  > fn f() {}
+  > ```
+
+- [MetaNameValueStr] --- The given string must ...
+  > [!EXAMPLE]
+  > ```rust
+  > #[example = "example"]
+  > fn f() {}
+  > ```
+
+- [MetaListNameValueStr] --- As with the [MetaNameValueStr] syntax, the given string must ...
+  > [!EXAMPLE]
+  > ```rust
+  > #[example(inner = "example")]
+  > fn f() {}
+  > ```
+
+----
+
 The syntax for the `example` attribute is:
 
 ```grammar,attributes
 @root ExampleAttribute -> `example` `(` ... `)`
 ```
+----
+
+r[PARENT.example.syntax.foo]
+The [MetaNameValueStr] form of the `example` attribute provides a way to specify the foo.
+
+> [!EXAMPLE]
+> ```rust
+> #[example = "example"]
+> fn some_meaningful_name() {}
+> ```
 
 r[PARENT.example.allowed-positions]
 Explain the valid positions where this attribute may be used.
@@ -61,10 +113,7 @@ The `example` attribute is allowed anywhere attributes are allowed.
 If there are unused attribute warnings, or if rustc is incorrectly accepting some positions, include a note about these.
 
 > [!NOTE]
-> `rustc` currently warns in other positions, but this may be rejected in the future.
-
-> [!NOTE]
-> `rustc` currently warns on some positions where it is ignored, but this may become an error in the future.
+> `rustc` ignores use in other positions but lints against it. This may become an error in the future.
 
 ----
 
@@ -73,29 +122,40 @@ Explain the behavior if the attribute is specified multiple times on an element.
 
 ----
 
-Duplicate instances of the `example` attribute on the same ... have no effect.
+The `example` attribute may be used any number of times on a form.
 
 ----
 
-The `example` attribute may only be specified once on a [THING].
+Using `example` more than once on a form has the same effect as using it once.
 
 ----
 
-Only the first instance of `example` on an item is honored. Subsequent `example` attributes are ignored.
+The `example` attribute may be used only once on ...
+
+----
+
+Only the first use of `example` on an item has effect.
+
+> [!NOTE]
+> `rustc` lints against any use following the first. This may become an error in the future.
 
 > [!NOTE]
-> `rustc` currently warns on subsequent duplicate `example` attributes. This may become an error in the future.
+> `rustc` lints against any use following the first with a future-compatibility warning. This may become an error in the future.
 
 ----
 
-Only the last instance of `example` on an item is honored. Previous `example` attributes are ignored.
+Only the last use of `example` on an item has effect.
 
 > [!NOTE]
-> `rustc` currently warns on preceding duplicate `example` attributes. This may become an error in the future.
+> `rustc` lints against any use preceding the last. This may become an error in the future.
 
----
+----
+
+Only the last use of `example` on an item is used to ...
+
+----
 
-If the `example` attribute is specified multiple times on an item, then the combination of all the specified values is used as ...explain how they are merged.
+If the `example` attribute is used more than once on an item, then the combination of all the specified values is used as ...explain how they are merged.
 
 ----