Update docs for setting verbose/quiet in config file or env var

Co-authored-by: Paul Moore <p.f.moore@gmail.com>
Co-authored-by: Prashant Sharma <prashantsharma161198@gmail.com>
Co-authored-by: Xavier Fernandez <xav.fernandez@gmail.com>

docs/html/user_guide.rst

@@ -442,6 +442,15 @@ and ``--no-cache-dir``, falsy values have to be used:
     no-compile = no
     no-warn-script-location = false
 
+For options which can be repeated like ``--verbose`` and ``--quiet``,
+a non-negative integer can be used to represent the level to be specified:
+
+.. code-block:: ini
+
+    [global]
+    quiet = 0
+    verbose = 2
+
 It is possible to append values to a section within a configuration file such as the pip.ini file.
 This is applicable to appending options like ``--find-links`` or ``--trusted-host``,
 which can be written on multiple lines:
@@ -488,6 +497,15 @@ is the same as calling::
 
     pip install --find-links=http://mirror1.example.com --find-links=http://mirror2.example.com
 
+Options that do not take a value, but can be repeated (such as ``--verbose``)
+can be specified using the number of repetitions, so::
+
+    export PIP_VERBOSE=3
+
+is the same as calling::
+
+    pip install -vvv
+
 .. note::
 
    Environment variables set to be empty string will not be treated as false.