Include Milestone reference syntax in Markdown documentation

doc/markdown/markdown.md

@@ -186,7 +186,7 @@ GFM will turn that reference into a link so you can navigate between them easily
 GFM will recognize the following:
 
 | input                  | references                   |
-|:-----------------------|:---------------------------|
+|:-----------------------|:---------------------------  |
 | `@user_name`           | specific user                |
 | `@group_name`          | specific group               |
 | `@all`                 | entire team                  |
@@ -196,6 +196,9 @@ GFM will recognize the following:
 | `~123`                 | label by ID                  |
 | `~bug`                 | one-word label by name       |
 | `~"feature request"`   | multi-word label by name     |
+| `%123`                 | milestone by ID              |
+| `%v1.23`               | one-word milestone by name   |
+| `%"release candidate"` | multi-word milestone by name |
 | `9ba12248`             | specific commit              |
 | `9ba12248...b19a04f5`  | commit range comparison      |
 | `[README](doc/README)` | repository file references   |
@@ -206,6 +209,7 @@ GFM also recognizes certain cross-project references:
 |:----------------------------------------|:------------------------|
 | `namespace/project#123`                 | issue                   |
 | `namespace/project!123`                 | merge request           |
+| `namespace/project%123`                 | milestone               |
 | `namespace/project$123`                 | snippet                 |
 | `namespace/project@9ba12248`            | specific commit         |
 | `namespace/project@9ba12248...b19a04f5` | commit range comparison |