DOC promote shallow copy in the docs (scikit-learn#16423)

* DOC promote shallow copy in the docs

* a shorter version

Author: adrinjalali

doc/developers/advanced_installation.rst

@@ -43,7 +43,7 @@ feature, code or documentation improvement).
    `scikit-learn repository <https://github.com/scikit-learn/scikit-learn>`_ on
    Github.::
 
-        git clone git://github.com/scikit-learn/scikit-learn.git
+        git clone git://github.com/scikit-learn/scikit-learn.git  # add --depth 1 if your connection is slow
         cd scikit-learn
 
    If you plan on submitting a pull-request, you should clone from your fork

doc/developers/contributing.rst

@@ -210,7 +210,7 @@ how to set up your git repository:
 3. Clone your fork of the scikit-learn repo from your GitHub account to your
    local disk::
 
-       $ git clone [email protected]:YourLogin/scikit-learn.git
+       $ git clone [email protected]:YourLogin/scikit-learn.git  # add --depth 1 if your connection is slow
        $ cd scikit-learn
 
 4. Install the development dependencies::