Document how to install provides_extras from local wheel file (#9698)

Author: johnthagen

docs/html/user_guide.rst

@@ -53,7 +53,7 @@ Specifiers`
 
       py -m pip install SomePackage            # latest version
       py -m pip install SomePackage==1.0.4     # specific version
-      py -m pip install 'SomePackage>=1.0.4'     # minimum version
+      py -m pip install 'SomePackage>=1.0.4'   # minimum version
 
 For more information and examples, see the :ref:`pip install` reference.
 
@@ -271,6 +271,26 @@ To install directly from a wheel archive:
 
       py -m pip install SomePackage-1.0-py2.py3-none-any.whl
 
+To include optional dependencies provided in the ``provides_extras``
+metadata in the wheel, you must add quotes around the install target
+name:
+
+.. tab:: Unix/macOS
+
+   .. code-block:: shell
+
+      python -m pip install './somepackage-1.0-py2.py3-none-any.whl[my-extras]'
+
+.. tab:: Windows
+
+   .. code-block:: shell
+
+      py -m pip install './somepackage-1.0-py2.py3-none-any.whl[my-extras]'
+
+.. note::
+
+    In the future, the ``path[extras]`` syntax may become deprecated. It is
+    recommended to use PEP 508 syntax wherever possible.
 
 For the cases where wheels are not available, pip offers :ref:`pip wheel` as a
 convenience, to build wheels for all your requirements and dependencies.