Add Section definition

Fixes https://github.com/gohugoio/hugoDocs/issues/409

content/content-management/sections.md

@@ -17,36 +17,49 @@ aliases: [/content/sections/]
 toc: true
 ---
 
+A **Section** is a collection of pages that gets defined based on the
+organization structure under the `content/` directory.
+
+By default, all the **first-level** directories under `content/` form their own
+sections (**root sections**).
+
+If a user needs to define a section `foo` at a deeper level, they need to create
+a directory named `foo` with an `_index.md` file (see [Branch Bundles][branch
+bundles] for more information).
+
+{{% note %}}
+A **section** cannot be defined or overridden by a front matter parameter -- it
+is strictly derived from the content organization structure.
+{{% /note %}}
 
 ## Nested Sections
 
 The sections can be nested as deeply as you need.
 
 ```bash
-blog
-├── funny-cats
-│   └── kittens
-│       └── _index.md
-└── tech
-    └── _index.md
+content
+└── blog        <-- Section, because first-level dir under content/
+    ├── funny-cats
+    │   ├── mypost.md
+    │   └── kittens         <-- Section, because contains _index.md
+    │       └── _index.md
+    └── tech                <-- Section, because contains _index.md
+        └── _index.md
 ```
 
-
-**The important part to understand is, that to make the section tree fully navigational, at least the lower-most section needs a content file. (e.g. `_index.md`).** 
-
+**The important part to understand is, that to make the section tree fully navigational, at least the lower-most section needs a content file. (e.g. `_index.md`).**
 
 {{% note %}}
-When we talk about a **section** in correlation with template selection, it is currently always the root section only (`/blog/funny/mypost/ => blog`). 
+When we talk about a **section** in correlation with template selection, it is
+currently always the *root section* only (`/blog/funny-cats/mypost/ => blog`).
 
 If you need a specific template for a sub-section, you need to adjust either the `type` or `layout` in front matter.
 {{% /note %}}
 
-
 ## Example: Breadcrumb Navigation
 
 With the available [section variables and methods](#section-page-variables-and-methods) you can build powerful navigation. One common example would be a partial to show Breadcrumb navigation:
 
-
 {{< code file="layouts/partials/breadcrumb.html" download="breadcrumb.html" >}}
 <ol  class="nav navbar-nav">
   {{ template "breadcrumbnav" (dict "p1" . "p2" .) }}
@@ -71,15 +84,14 @@ Also see [Page Variables](/variables/page/).
 
 ## Content Section Lists
 
-Hugo will automatically create pages for each section root that list all of the content in that section. See the documentation on [section templates][] for details on customizing the way these pages are rendered.
+Hugo will automatically create pages for each *root section* that list all of the content in that section. See the documentation on [section templates][] for details on customizing the way these pages are rendered.
 
 ## Content *Section* vs Content *Type*
 
-By default, everything created within a section will use the [content type][] that matches the root section name. For example, Hugo will assume that `posts/post-1.md` has a `posts` content type. If you are using an [archetype][] for your posts section, Hugo will generate front matter according to what it finds in `archetypes/posts.md`.
+By default, everything created within a section will use the [content `type`][content type] that matches the *root section* name. For example, Hugo will assume that `posts/post-1.md` has a `posts` content `type`. If you are using an [archetype][] for your `posts` section, Hugo will generate front matter according to what it finds in `archetypes/posts.md`.
 
 [archetype]: /content-management/archetypes/
 [content type]: /content-management/types/
 [directory structure]: /getting-started/directory-structure/
 [section templates]: /templates/section-templates/
-
-
+[branch bundles]: /content-management/page-bundles/#branch-bundles