Update frontmatter docs (#786)

* Update frontmatter docs

* Add some text

Author: reakaleek

docs/syntax/frontmatter.md

@@ -1,10 +1,34 @@
 # Frontmatter
 
-Every Markdown file referenced in the TOC may optionally define a frontmatter block. Frontmatter is YAML-formatted metadata about a page, at the beginning of each file.
+Every Markdown file referenced in the TOC may optionally define a frontmatter block.
+Frontmatter is YAML-formatted metadata about a page, at the beginning of each file
+and wrapped by `---` lines.
 
-Supported frontmatter includes:
+In the frontmatter block, you can define the following fields:
 
-| Frontmatter         | Learn more                  |
-| ------------------- | --------------------------- |
-| `navigation_title`  | See [](./titles.md)    |
-| `applies`           | See [](./applies.md) |
+```yaml
+---
+navigation_title: This is the navigation title <1>
+description: This is a description of the page <2>
+applies_to: <3>
+  serverless: all
+---
+```
+1. [`navigation_title`](#navigation-title)
+2. [`description`](#description)
+3. [`applies_to`](#applies-to)
+
+## Navigation Title
+See [](./titles.md)
+
+## Description
+
+Use the `description` frontmatter to set the description meta tag for a page. 
+This helps search engines and social media.
+It also sets the `og:description` and `twitter:description` meta tags.
+
+The `description` frontmatter is a string, recommended to be around 150 characters. If you don't set a `description`, 
+it will be generated from the first few paragraphs of the page until it reaches 150 characters.
+
+## Applies to
+See [](./applies.md)