feature #6230 Add annotation to glossary (rebased) (DerStoffel)

This PR was submitted for the master branch but it was merged into the 2.3 branch instead (closes #6230).

Discussion
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Add annotation to glossary (rebased)

Commits
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914523c Add annotation to glossary (rebased)

glossary.rst

@@ -125,6 +125,11 @@ Glossary
         Symfony's configuration files. See the :doc:`/components/yaml/introduction`
         chapter.
 
+   Annotation
+         Annotations are metadata written alongside your code. They can either be explanatory and will be
+         ignored during execution or add functionality to the line of code directly below as a means of
+         configuration. For example, the annotation ``@var`` describes the type of a variable, whereas in
+         Symfony2 ``@Assert`` can add validation to a member variable of a class (see :doc:`/book/validation` chapter).
 
 .. _`service-oriented architecture`: https://wikipedia.org/wiki/Service-oriented_architecture
 .. _`HTTP Wikipedia`: https://en.wikipedia.org/wiki/Hypertext_Transfer_Protocol