Documentation/pretty-{formats,options}: better reference for "format:<string>"

In "git help log" (and friends) it's not easy to find the possible
placeholder for <string> for the "--pretty=format:<string>" option
to git log.

This patch makes the placeholder easier to find by adding a reference
to the "PRETTY FORMATS" section and repeating the "format:<string>"
phrase.

Signed-off-by: Nazri Ramliy <ayiehere@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/pretty-formats.txt

@@ -76,9 +76,9 @@ displayed in full, regardless of whether --abbrev or
 true parent commits, without taking grafts nor history
 simplification into account.
 
-* 'format:'
+* 'format:<string>'
 +
-The 'format:' format allows you to specify which information
+The 'format:<string>' format allows you to specify which information
 you want to show. It works a little bit like printf format,
 with the notable exception that you get a newline with '%n'
 instead of '\n'.

Documentation/pretty-options.txt

@@ -3,8 +3,9 @@
 
 	Pretty-print the contents of the commit logs in a given format,
 	where '<format>' can be one of 'oneline', 'short', 'medium',
-	'full', 'fuller', 'email', 'raw' and 'format:<string>'.
-	When omitted, the format defaults to 'medium'.
+	'full', 'fuller', 'email', 'raw' and 'format:<string>'.  See
+	the "PRETTY FORMATS" section for some additional details for each
+	format.  When omitted, the format defaults to 'medium'.
 +
 Note: you can specify the default pretty format in the repository
 configuration (see linkgit:git-config[1]).