Add links to READMEs for reusables and variables (#17352)

Also update sentences to adhere to style guide

Author: ethanpalm

contributing/content-markup-reference.md

@@ -98,10 +98,10 @@ These instructions are pertinent to Windows users.
 {% endlinux %}
 ```
 
-You can define a default platform in the frontmatter, see the [content README](../content/README.md#defaultplatform).
+You can define a default platform in the frontmatter. For more information, see the [content README](../content/README.md#defaultplatform).
 
 ## Reusable and variable strings of text
 
 Reusable strings (commonly called content references or conrefs) contain content that’s used in more than one place in our documentation and allow us to change the content in a single location rather than every place the string appears.
 
-For longer strings, we use [reusables](), and for shorter strings, we use [variables](). See each linked README for usage instructions.
+For longer strings, we use reusables, and for shorter strings, we use variables. For more information about reusables, see the [reusables README](../data/reusables/README.md). For more information about variables, see the [variables README](../data/variables/README.md).