git-for-each-ref: improve the documentation on scripting modes

When reading the synopsis for git-for-each-ref it is easy to miss
the obvious power of --shell and family.  Call this feature out in
the primary paragragh.  Also add more description to the examples
to indicate which features we are demonstrating.  Finally add a
very simple eval based example in addition to the very complex one
to give a gentler introduction.
Signed-off-by: NAndy Whitcroft <apw@shadowen.org>
Signed-off-by: NJunio C Hamano <junkio@cox.net>

Documentation/git-for-each-ref.txt

@@ -17,7 +17,7 @@ according to the given `<format>`, after sorting them according
 to the given set of `<key>`s.  If `<max>` is given, stop after
 showing that many refs.  The interporated values in `<format>`
 can optionally be quoted as string literals in the specified
-host language.
+host language allowing their direct evaluation in that language.
 
 OPTIONS
 -------
@@ -97,7 +97,8 @@ returns an empty string instead.
 EXAMPLES
 --------
 
-Show the most recent 3 tagged commits::
+An example directly producing formatted text.  Show the most recent
+3 tagged commits::
 
 ------------
 #!/bin/sh
@@ -112,7 +113,23 @@ Ref: %(*refname)
 ' 'refs/tags'
 ------------
 
-A bit more elaborate report on tags::
+
+A simple example showing the use of shell eval on the output,
+demonstrating the use of --shell.  List the prefixes of all heads::
+------------
+#!/bin/sh
+
+git-for-each-ref --shell --format="ref=%(refname)" refs/heads | \
+while read entry
+do
+	eval "$entry"
+	echo `dirname $ref`
+done
+------------
+
+
+A bit more elaborate report on tags, demonstrating that the format
+may be an entire script::
 ------------
 #!/bin/sh
 
@@ -156,7 +173,7 @@ Its message reads as:
 	fi
 '
 
-eval=`git-for-each-ref -s --format="$fmt" \
+eval=`git-for-each-ref --shell --format="$fmt" \
 	--sort='*objecttype' \
 	--sort=-taggerdate \
 	refs/tags`