Replace incorrect statement about triggering installation of dependencies to...

Replace incorrect statement about triggering installation of dependencies to indicate that the handling is mostly implementation specific. Ref pypa/setuptools#1471.

docs/userguide/dependency_management.rst

@@ -262,8 +262,12 @@ First is the console_scripts entry point:
         }
     )
 
-When the script ``rst2pdf`` is run, it will trigger the installation of
-the two dependencies ``PDF`` maps to.
+This syntax indicates that the entry point (in this case a console script)
+is only valid when the PDF extra is installed. It is up to the installer
+to determine how to handle the situation where PDF was not indicated
+(e.g. omit the console script, provide a warning when attempting to load
+the entry point, assume the extras are present and let the implementation
+fail later).
 
 The second use case is that other package can use this "extra" for their
 own dependencies. For example, if "Project-B" needs "project A" with PDF support