Add macOS to description_guidelines.md

Also fix the mappings to actually appear when viewing the doc as markdown.

Change-Id: Ie68a1651a95b2876dcea1fe31a7aea54c872d2b3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2350250Reviewed-by: Owen Min <zmin@chromium.org>
Commit-Queue: Chris Sharp <csharp@chromium.org>
Cr-Commit-Position: refs/heads/master@{#797369}

docs/enterprise/description_guidelines.md

 To ensure consistency in policy descriptions, the following is a mapping of
 how various product names and the like should be referenced.
 
-Chrome: <ph name="PRODUCT_NAME">$1<ex>Google Chrome</ex></ph>
-Chrome OS: <ph name="PRODUCT_OS_NAME">$2<ex>Google Chrome OS</ex></ph>
-Linux: <ph name="LINUX_OS_NAME">Linux</ph>
-Internet Explorer: <ph name="IE_PRODUCT_NAME">Internet® Explorer®</ph>
-Google Cast: <ph name="PRODUCT_NAME">Google Cast</ph>
-Google Drive: <ph name="GOOGLE_DRIVE_NAME">Google Drive</ph>
-Windows: <ph name="MS_WIN_NAME">Microsoft® Windows®</ph>
-Microsoft ActiveDirectory: <ph name="MS_AD_NAME">Microsoft® Active Directory®</ph>
+* Chrome: `<ph name="PRODUCT_NAME">$1<ex>Google Chrome</ex></ph>`
+* Chrome OS: `<ph name="PRODUCT_OS_NAME">$2<ex>Google Chrome OS</ex></ph>`
+* Linux: `<ph name="LINUX_OS_NAME">Linux</ph>`
+* Internet Explorer: `<ph name="IE_PRODUCT_NAME">Internet® Explorer®</ph>`
+* Google Cast: `<ph name="PRODUCT_NAME">Google Cast</ph>`
+* Google Drive: `<ph name="GOOGLE_DRIVE_NAME">Google Drive</ph>`
+* macOS: `<ph name="MAC_OS_NAME">macOS</ph>`
+* Windows: `<ph name="MS_WIN_NAME">Microsoft® Windows®</ph>`
+* Microsoft ActiveDirectory: `<ph name="MS_AD_NAME">Microsoft® Active Directory®</ph>`