Add documentation around plural control flow

Previously we have had cases of the `n__` method being used to control
which different strings should show up in the singular and plural cases
for a string. This doesn't work in languages such as Chinese Simplified,
as this language only has one target plural form - which means the
translator needs to determine which string should show up.

This is particularly an issue when different variables are used in each
version of the string, as variables present in the source string for
one case may not show up in the target string for the chosen plural form
from the translators - which will cause linting to fail.

doc/development/i18n/externalization.md

@@ -195,6 +195,31 @@ For example use `%{created_at}` in Ruby but `%{createdAt}` in JavaScript. Make s
   // => When x == 2: 'Last 2 days'
   ```
 
+The `n_` method should only be used to fetch pluralized translations of the same
+string, not to control the logic of showing different strings for different
+quantities. Some languages have different quantities of target plural forms -
+Chinese (simplified), for example, has only one target plural form in our
+translation tool. This means the translator would have to choose to translate
+only one of the strings and the translation would not behave as intended in the
+other case.
+
+For example, prefer to use:
+
+```ruby
+if selected_projects.one?
+  selected_projects.first.name
+else
+  n__("Project selected", "%d projects selected", selected_projects.count)
+end
+```
+
+rather than:
+
+```ruby
+# incorrect usage example
+n_("%{project_name}", "%d projects selected", count) % { project_name: 'GitLab' }
+```
+
 ### Namespaces
 
 Sometimes you need to add some context to the text that you want to translate