Update docs about composer-packaged themes to include naming convention

guides/v2.1/extension-dev-guide/package/package_module.md

@@ -27,7 +27,7 @@ The `composer.json` uses [Composer's generic schema](https://getcomposer.org/doc
 
 Element | Description
 --- | ---
-`name` | A fully-qualified component name, in the format `<vendor-name>/<component-name>`. All letters must be in lowercase. Use dashes in the `<component-name>` to separate words.
+`name` | A fully-qualified component name, in the format `<vendor-name>/<component-name>`. All letters must be in lowercase. Use dashes in the `<component-name>` to separate words. Themes must use the format `<vendor-name>/theme-<area>-<theme-name>`.
 `type` | For modules, this value must be set to `magento2-module`. Other possible types are `metapackage`, `magento2-theme`, and `magento2-language`.
 `autoload` | Specify necessary information to be loaded, such as [registration.php]({{ page.baseurl }}/extension-dev-guide/build/component-registration.html). For more information, see [Autoloading](https://getcomposer.org/doc/01-basic-usage.md#autoloading) from Composer.
 

guides/v2.1/frontend-dev-guide/themes/theme-structure.md

@@ -230,7 +230,7 @@ The directories and files structure described below is the most extended one. It
       </td>
       <td colspan="1">optional</td>
       <td colspan="1">
-        Describes the theme dependencies and some meta-information. Will be here if your theme is a Composer package.
+        Describes the theme dependencies and some meta-information. Will be here if your theme is a Composer package. The "name" field must be in the format <code>"&lt;vendor-name&gt;/theme-&lt;area&gt;-&lt;theme-name&gt;"</code>.
       </td>
     </tr>
     <tr>

guides/v2.2/frontend-dev-guide/themes/theme-structure.md

@@ -230,7 +230,7 @@ The directories and files structure described below is the most extended one. It
       </td>
       <td colspan="1">optional</td>
       <td colspan="1">
-        Describes the theme dependencies and some meta-information. Will be here if your theme is a Composer package.
+        Describes the theme dependencies and some meta-information. Will be here if your theme is a Composer package. The "name" field must be in the format <code>"&lt;vendor-name&gt;/theme-&lt;area&gt;-&lt;theme-name&gt;"</code>.
       </td>
     </tr>
     <tr>

guides/v2.3/frontend-dev-guide/themes/theme-structure.md

@@ -230,7 +230,7 @@ The directories and files structure described below is the most extended one. It
       </td>
       <td colspan="1">optional</td>
       <td colspan="1">
-        Describes the theme dependencies and some meta-information. Will be here if your theme is a Composer package.
+        Describes the theme dependencies and some meta-information. Will be here if your theme is a Composer package. The "name" field must be in the format <code>"&lt;vendor-name&gt;/theme-&lt;area&gt;-&lt;theme-name&gt;"</code>.
       </td>
     </tr>
     <tr>