doc: clarify syntax for %C(auto,...) in pretty formats

The manual correctly describes the syntax with `auto,` but the
trailing `,` is hard to spot in a terminal.  The HTML format does not
have this problem.  Adding an example helps both worlds.

Signed-off-by: Andreas Heiduk <asheiduk@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/pretty-formats.txt

@@ -174,11 +174,12 @@ endif::git-rev-list[]
 - '%Creset': reset color
 - '%C(...)': color specification, as described under Values in the
   "CONFIGURATION FILE" section of linkgit:git-config[1];
-  adding `auto,` at the beginning will emit color only when colors are
-  enabled for log output (by `color.diff`, `color.ui`, or `--color`, and
-  respecting the `auto` settings of the former if we are going to a
-  terminal). `auto` alone (i.e. `%C(auto)`) will turn on auto coloring
-  on the next placeholders until the color is switched again.
+  adding `auto,` at the beginning (e.g. `%C(auto,red)`) will emit
+  color only when colors are enabled for log output (by `color.diff`,
+  `color.ui`, or `--color`, and respecting the `auto` settings of the
+  former if we are going to a terminal). `auto` alone (i.e.
+  `%C(auto)`) will turn on auto coloring on the next placeholders
+  until the color is switched again.
 - '%m': left (`<`), right (`>`) or boundary (`-`) mark
 - '%n': newline
 - '%%': a raw '%'