user-manual: mention "..." in "Generating diffs", etc.

We should mention the use of the "..." syntax for git-diff here.  The
note about the difference between diff and the combined output of
git-format-patch then no longer fits so well, so remove it.  Add a
reference to the git-format-patch[1] manpage.

Signed-off-by: J. Bruce Fields <bfields@citi.umich.edu>

Documentation/user-manual.txt

@@ -658,16 +658,23 @@ gitlink:git-diff[1]:
 $ git diff master..test
 -------------------------------------------------
 
-Sometimes what you want instead is a set of patches:
+That will produce the diff between the tips of the two branches.  If
+you'd prefer to find the diff from their common ancestor to test, you
+can use three dots instead of two:
+
+-------------------------------------------------
+$ git diff master...test
+-------------------------------------------------
+
+Sometimes what you want instead is a set of patches; for this you can
+use gitlink:git-format-patch[1]:
 
 -------------------------------------------------
 $ git format-patch master..test
 -------------------------------------------------
 
 will generate a file with a patch for each commit reachable from test
-but not from master.  Note that if master also has commits which are
-not reachable from test, then the combined result of these patches
-will not be the same as the diff produced by the git-diff example.
+but not from master.
 
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