Document enabling --no-warn-script-location with falsy values

Closes pypa#6209

docs/html/user_guide.rst

@@ -396,8 +396,8 @@ set like this:
     ignore-installed = true
     no-dependencies = yes
 
-To enable the boolean options ``--no-compile`` and ``--no-cache-dir``, falsy
-values have to be used:
+To enable the boolean options ``--no-compile``, ``--no-warn-script-location``
+and ``--no-cache-dir``, falsy values have to be used:
 
 .. code-block:: ini
 
@@ -406,6 +406,7 @@ values have to be used:
 
     [install]
     no-compile = no
+    no-warn-script-location = false
 
 Appending options like ``--find-links`` can be written on multiple lines:
 

src/pip/_internal/commands/install.py

@@ -215,7 +215,8 @@ def __init__(self, *args, **kw):
             action="store_false",
             dest="warn_script_location",
             default=True,
-            help="Do not warn when installing scripts outside PATH",
+            help="Do not warn when installing scripts outside PATH. Must use "
+                 "falsy value to enable.",
         )
         cmd_opts.add_option(
             "--no-warn-conflicts",