Mention "-m" in the main mark docs

Author: nicoddemus

doc/en/example/markers.rst

@@ -6,6 +6,8 @@ Working with custom markers
 
 Here are some example using the :ref:`mark` mechanism.
 
+.. _`mark run`:
+
 Marking test functions and selecting them for a run
 ----------------------------------------------------
 

doc/en/mark.rst

@@ -15,8 +15,10 @@ some builtin markers, for example:
   to the same test function.
 
 It's easy to create custom markers or to apply markers
-to whole test classes or modules. See :ref:`mark examples` for examples
-which also serve as documentation.
+to whole test classes or modules. Those markers can be used by plugins, and also
+are commonly used to :ref:`select tests <mark run>` on the command-line with the ``-m`` option.
+
+See :ref:`mark examples` for examples which also serve as documentation.
 
 .. note::