Mention cross-project labels in markdown documentation

app/models/label.rb

@@ -48,7 +48,11 @@ class Label < ActiveRecord::Base
     '~'
   end
 
+  ##
   # Pattern used to extract label references from text
+  #
+  # This pattern supports cross-project references.
+  #
   def self.reference_pattern
     %r{
       (#{Project.reference_pattern})?

doc/markdown/markdown.md

@@ -207,6 +207,7 @@ GFM also recognizes certain cross-project references:
 | `namespace/project$123`                 | snippet                 |
 | `namespace/project@9ba12248`            | specific commit         |
 | `namespace/project@9ba12248...b19a04f5` | commit range comparison |
+| `namespace/project~"Some label"`        | issues with given label |
 
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