replace fenced code blocks with AsciiDoc source blocks

modules/attributes/pages/roles.adoc

@@ -100,13 +100,15 @@ The HTML source code that is output from <<ex-role-dot>> is shown below.
 ////
 Using the shorthand notation, an id can also be specified:
 
-```
+[source]
+----
 [#value.value]`monospace text`
-```
+----
 
 which produces:
 
-```html
+[source,html]
+----
 <a id="value"></a><code class="value">monospace text</code>
-```
+----
 ////

modules/attributes/pages/unresolved-references.adoc

@@ -79,9 +79,10 @@ The option `skip` doesn't make sense here since the statement is not intended to
 
 Consider the following declaration:
 
-```
+[source]
+----
 {set:name!}
-```
+----
 
 Depending on whether `attribute-undefined` is `drop` or `drop-line`, either the statement or the line that contains it will be discarded.
 It's reasonable to stick with the compliant behavior, drop-line, in this case.

modules/text/pages/monospace.adoc

@@ -149,9 +149,10 @@ Since we're swapping behavior between two existing formatting marks, we had to i
 
 **IMPORTANT:** I want to reiterate that you have the option of enabling legacy mode if you don't want to deal with migration. Legacy mode will be supported for the foreseeable future to avoid unnecessary disruption. If that's the option you want to take, simply add the following document attribute at the start of your document, or wherever you want to enable legacy mode.
 
-```
+[source]
+----
 :compat-mode:
-```
+----
 
 If you want to begin migration to the modern syntax, read on.