Add a note to the pip-install docs.

This was suggested by @xavfernandez in the review comments.

docs/reference/pip_install.rst

@@ -374,6 +374,10 @@ Passing branch names, a commit hash or a tag name is possible like so::
     [-e] git://git.example.com/MyProject.git@v1.0#egg=MyProject
     [-e] git://git.example.com/MyProject.git@da39a3ee5e6b4b0d3255bfef95601890afd80709#egg=MyProject
 
+When passing a commit hash, specifying a full hash is preferable to a partial
+hash because a full hash allows pip to operate more efficiently (e.g. by
+making fewer network calls).
+
 Mercurial
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