Add doc notes about using quote marks on Mac when installing optional et al (#1916)

Author: kandersolar

docs/sphinx/source/contributing.rst

@@ -261,7 +261,7 @@ First, install the ``doc`` dependencies specified in the
 `setup.py <https://github.com/pvlib/pvlib-python/blob/main/setup.py>`_.
 An easy way to do this is with::
 
-    pip install pvlib[doc]
+    pip install pvlib[doc]    # on Mac:  pip install "pvlib[doc]"
 
 Note: Anaconda users may have trouble using the above command to update an
 older version of docutils. If that happens, you can update it with ``conda``

docs/sphinx/source/user_guide/installation.rst

@@ -71,7 +71,7 @@ non-editable way, use one of the following commands to install pvlib-python::
 
     # get pvlib and optional dependencies from the Python Package Index
     # another option if you know what you are doing
-    pip install pvlib[optional]
+    pip install pvlib[optional]  # on Mac:  pip install "pvlib[optional]"
 
 .. note::
 
@@ -239,7 +239,7 @@ The Anaconda distribution includes most of the above packages.
 
 Alternatively, users may install all optional dependencies using
 
-    pip install pvlib[optional]
+    pip install pvlib[optional]  # on Mac:  pip install "pvlib[optional]"
 
 
 .. _nrelspa: