Clarify use of composer.json

Author: stevjohnson

guides/v2.0/config-guide/bootstrap/magento-how-to-set.md

@@ -63,6 +63,11 @@ bash shell example for CentOS:
 
 	export MAGE_PROFILER=firebug
 
+<div class="bs-callout bs-callout-info" id="info">
+<span class="glyphicon-class">
+  <p>If a <code>PHP Fatal error</code> displays in the browser after you set a profiler value, restart your web server. The reason might be related to PHP bytecode caching, which caches bytecodes and PHP classpaths.</p></span>
+</div>
+
 <h2 id="mode-specify-web">Specifying a parameter value</h2>
 This section discusses how to specify the mode for either Apache or nginx.
 

guides/v2.0/extension-dev-guide/build/composer-integration.md

@@ -22,7 +22,13 @@ redirect_from:
 
 [Composer](https://getcomposer.org/) is a dependency manager for PHP. Magento 2 uses Composer to package components and product editions. 
 
-Some third-party components that the Magento system uses might not be present in the code base. Instead, they are listed as dependencies in the root `composer.json` file. 
+Some third-party components that the Magento system uses might not be present in the code base. Instead, they are listed as dependencies in the root `composer.json` file. In addition, the [Component Manager]({{ site.gdeurl }}comp-mgr/compman-start.html) looks for a `composer.json` in a component's root directory and can perform actions on the component and its dependencies. 
+
+In particular:
+
+*	If a component has `composer.json` *and* the component was installed using Composer (including from packagist, the Magento Marketplace, or other source), the Component Manager can update, uninstall, enable, or disable the component.
+*	If the component has `composer.json` but was *not* installed using Composer (for example, custom code a developer wrote), Component Manager can still enable or disable the component.
+*	We strongly recommend you include `composer.json` in your component's root directory whether or not you intend to distribute it to other Magento merchants.
 
 ## Overview of composer.json files {#composerjson-overview}
 Certain Magento components and product editions are represented with `composer.json` files. 

guides/v2.0/extension-dev-guide/build/create_component.md

@@ -46,6 +46,14 @@ The smallest working module.xml file would look something like this:
 ##Add the components `composer.json` file {#add-composer-json}
 `composer.json` provides a component name and also specifies component dependencies.
 
+In addition, the [Component Manager]({{ site.gdeurl }}comp-mgr/compman-start.html) looks for a `composer.json` in a component's root directory and can perform actions on the component and its dependencies. 
+
+In particular:
+
+* If a component has `composer.json` *and* the component was installed using Composer (including from packagist, the Magento Marketplace, or other source), the Component Manager can update, uninstall, enable, or disable the component.
+* If the component has `composer.json` but was *not* installed using Composer (for example, custom code a developer wrote), Component Manager can still enable or disable the component.
+* We strongly recommend you include `composer.json` in your component's root directory whether or not you intend to distribute it to other Magento merchants.
+
 A sample follows:
 
 {% highlight JSON %}