Add help document describing wiki linking behavior.

app/views/projects/wikis/_form.html.haml

@@ -20,6 +20,8 @@
       .help-block
         To link to a (new) page, simply type
         %code [Link Title](page-slug)
+        \. More examples are in the
+        = link_to 'documentation', help_page_path(category: 'markdown', file: 'wiki', format: 'md')
         \.
   .form-group
     = f.label :commit_message, class: 'control-label'

doc/markdown/markdown.md

@@ -28,6 +28,10 @@
 * [Line Breaks](#line-breaks)
 * [Tables](#tables)
 
+**[Wiki-Specific Markdown](wiki.md)**
+
+* [Links](wiki.md#links-to-other-wiki-pages)
+
 **[References](#references)**
 
 ## GitLab Flavored Markdown (GFM)

doc/markdown/wiki.md

+# Wiki-Specific Markdown
+
+## Table of Contents
+
+* [Links to Other Wiki Pages](#links-to-other-wiki-pages)
+  * [Direct Page Link](#direct-page-link)
+  * [Direct File Link](#direct-file-link)
+  * [Hierarchical Link](#hierarchical-link)
+  * [Root Link](#root-link)
+
+## Links to Other Wiki Pages
+
+You can link to other pages on your wiki in a few different ways.
+
+### Direct Page Link
+
+A link which just includes the slug for a page will point to that page, _at the base level of the wiki_.
+
+1. This snippet would link to a `documentation` page at the root of your wiki.
+
+```markdown
+[Link to Documentation](documentation)
+```
+
+### Direct File Link
+
+Links with a file extension point to that file, _relative to the current page_.
+
+1. If this snippet was placed on a page at `<your_wiki>/documentation/related`, it would link to `<your_wiki>/documentation/file.md`.
+
+	```markdown
+	[Link to File](file.md)
+	```
+
+### Hierarchical Link
+
+A link can be constructed relative to the current wiki page using `./<page>`, `../<page>`, etc.
+
+1. If this snippet was placed on a page at `<your_wiki>/documentation/main`, it would link to `<your_wiki>/documentation/related`.
+
+	```markdown
+	[Link to Related Page](./related)
+	```
+
+1. If this snippet was placed on a page at `<your_wiki>/documentation/related/content`, it would link to `<your_wiki>/documentation/main`.
+
+	```markdown
+	[Link to Related Page](../main)
+	```
+
+1. If this snippet was placed on a page at `<your_wiki>/documentation/main`, it would link to `<your_wiki>/documentation/related.md`.
+
+	```markdown
+	[Link to Related Page](./related.md)
+	```
+
+1. If this snippet was placed on a page at `<your_wiki>/documentation/related/content`, it would link to `<your_wiki>/documentation/main.md`.
+
+	```markdown
+	[Link to Related Page](../main.md)
+	```
+
+### Root Link
+
+A link starting with a `/` is relative to the wiki root, for non-file links.
+
+1. This snippet links to `<wiki_root>/documentation`
+
+	```markdown
+	[Link to Related Page](/documentation)
+	```
+
+1. This snippet links to `<wiki_root>/miscellaneous.md`
+
+	```markdown
+	[Link to Related Page](/miscellaneous.md)
+	```