Merge branch 'add-notes-style' into 'master'

Update styleguide.md to add Notes usage guidelines

See merge request gitlab-org/gitlab-ce!29429

doc/development/documentation/styleguide.md

@@ -577,7 +577,7 @@ nicely on different mobile devices.
 
 ## Alert boxes
 
-Whenever you want to call the attention to a particular sentence,
+Whenever you need to call special attention to particular sentences,
 use the following markup for highlighting.
 
 _Note that the alert boxes only work for one paragraph only. Multiple paragraphs,
@@ -585,6 +585,23 @@ lists, headers, etc will not render correctly. For multiple lines, use blockquot
 
 ### Note
 
+Notes catch the eye of most readers, and therefore should be used very sparingly.
+In most cases, content considered for a note should be included:
+
+- As just another sentence in the previous paragraph or the most-relevant paragraph.
+- As its own standalone paragraph.
+- As content under a new subheading that introduces the topic, making it more visible/findable.
+
+#### When to use
+
+Use a note when there is a reason that most or all readers who browse the
+section should see the content. That is, if missed, it’s likely to cause 
+major trouble for a minority of users or significant trouble for a majority
+of users.
+
+Weigh the costs of distracting users to whom the content is not relevant against
+the cost of users missing the content if it were not expressed as a note.
+
 ```md
 NOTE: **Note:**
 This is something to note.