Make it clearer what "make repo" is meant for.

Closes #3330.

Makefile

@@ -26,6 +26,7 @@ TMPDIR = .repo_tmp
 	rm -rf $(TMPDIR)
 	git ls-files -d | xargs git checkout --
 
+# Create a git repo from an unpacked source directory.
 repo: .git
 
 

README.rst

@@ -51,8 +51,9 @@ Get the full source history:
 
 Note that Cython used to ship the full version control repository in its source
 distribution, but no longer does so due to space constraints.  To get the
-full source history, make sure you have git installed, then step into the
-base directory of the Cython source distribution and type::
+full source history from a downloaded source archive, make sure you have git
+installed, then step into the base directory of the Cython source distribution
+and type::
 
     make repo