Add info about mdl to documentation

doc/development/documentation/index.md

@@ -488,7 +488,9 @@ Currently, the following tests are in place:
    that all cURL examples in API docs use the full switches. It's recommended
    to [check locally](#previewing-the-changes-live) before pushing to GitLab by executing the command
    `bundle exec nanoc check internal_links` on your local
-   [`gitlab-docs`](https://gitlab.com/gitlab-org/gitlab-docs) directory.
+   [`gitlab-docs`](https://gitlab.com/gitlab-org/gitlab-docs) directory. In addition,
+   `docs-lint` also runs [markdownlint](styleguide.md#markdown-rules) to ensure the
+   markdown is consistent across all documentation.
 1. [`ee_compat_check`](../automatic_ce_ee_merge.md#avoiding-ce-ee-merge-conflicts-beforehand) (runs on CE only):
    When you submit a merge request to GitLab Community Edition (CE),
    there is this additional job that runs against Enterprise Edition (EE)
@@ -559,15 +561,16 @@ A file with `proselint` configuration must be placed in a
 #### `markdownlint`
 
 `markdownlint` checks that certain rules ([example](https://github.com/DavidAnson/markdownlint/blob/master/README.md#rules--aliases))
- are followed for Markdown syntax.
- Our [Documentation Style Guide](styleguide.md) and [Markdown Guide](https://about.gitlab.com/handbook/product/technical-writing/markdown-guide/)
- elaborate on which choices must be made when selecting Markdown syntax for
- GitLab documentation. This tool helps catch deviations from those guidelines.
+are followed for Markdown syntax. Our [Documentation Style Guide](styleguide.md) and
+[Markdown Guide](https://about.gitlab.com/handbook/product/technical-writing/markdown-guide/)
+elaborate on which choices must be made when selecting Markdown syntax for GitLab
+documentation. This tool helps catch deviations from those guidelines, and matches the
+tests run on the documentation by [`docs-lint`](#testing).
 
 `markdownlint` can be used [on the command line](https://github.com/igorshubovych/markdownlint-cli#markdownlint-cli--),
- either on a single Markdown file or on all Markdown files in a project. For example, to run
- `markdownlint` on all documentation in the [`gitlab-ce` project](https://gitlab.com/gitlab-org/gitlab-ce),
- run the following commands from within the `gitlab-ce` project:
+either on a single Markdown file or on all Markdown files in a project. For example, to run
+`markdownlint` on all documentation in the [`gitlab-ce` project](https://gitlab.com/gitlab-org/gitlab-ce),
+run the following commands from within the `gitlab-ce` project:
 
 ```sh
 cd doc
@@ -597,7 +600,7 @@ The following sample `markdownlint` configuration modifies the available default
   "line-length": false,
   "no-trailing-punctuation": false,
   "ol-prefix": { "style": "one" },
-  "blanks-around-fences": false,
+  "blanks-around-fences": true,
   "no-inline-html": {
     "allowed_elements": [
       "table",
@@ -612,11 +615,15 @@ The following sample `markdownlint` configuration modifies the available default
       "a",
       "strong",
       "i",
-      "div"
+      "div",
+      "b"
     ]
   },
   "hr-style": { "style": "---" },
-  "fenced-code-language": false
+  "code-block-style": { "style": "fenced" },
+  "fenced-code-language": false,
+  "no-duplicate-header": { "allow_different_nesting": true },
+  "commands-show-output": false
 }
 ```
 

doc/development/documentation/styleguide.md

@@ -107,6 +107,22 @@ Hard-coded HTML is valid, although it's discouraged to be used while we have `/h
 - Special styling is required.
 - Reviewed and approved by a technical writer.
 
+### Markdown Rules
+
+GitLab ensures that the Markdown used across all documentation is consistent, as
+well as easy to review and maintain, by testing documentation changes with
+[Markdownlint (mdl)](https://github.com/markdownlint/markdownlint). This lint test
+checks many common problems with Markdown, and fails when any document has an issue
+with Markdown formatting that may cause the page to render incorrectly within GitLab.
+It will also fail when a document is using non-standard Markdown (which may render
+correctly, but is not the current standard in GitLab documentation).
+
+Each formatting issue that mdl checks has an associated [rule](https://github.com/markdownlint/markdownlint/blob/master/docs/RULES.md),
+and the rules that are currently enabled for GitLab documentation are listed in the
+[`.mdlrc.style`](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/.mdlrc.style) file.
+Configuration options are set in the [`.mdlrc`](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/.mdlrc.style)
+file.
+
 ## Structure
 
 ### Organize by topic, not by type