[wwwdocs] Add reference to pp_format to Coding Conventions

[Jonathan Wakely via Gcc-patches]

Finding the docs for the GCC-specific diagnostic formats isn't easy.
This might help.

OK for wwwdocs?

---
 htdocs/codingconventions.html | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/htdocs/codingconventions.html b/htdocs/codingconventions.html
index e4d30510..f1d1f165 100644
--- a/htdocs/codingconventions.html
+++ b/htdocs/codingconventions.html
@@ -333,7 +333,8 @@ code token.</li>
 <li>Diagnostics using the GCC diagnostic functions should generally
 use the GCC-specific formats such as <code>%qs</code> or
 <code>%&lt;</code> and <code>%&gt;</code> for quoting and
-<code>%m</code> for <code>errno</code> numbers.</li>
+<code>%m</code> for <code>errno</code> numbers. See
+<tt>pp_format</tt> in <tt>pretty-print.cc</tt> for supported formats.</li>
 
 <li>Identifiers should generally be formatted with <code>%E</code> or
 <code>%qE</code>; use of <code>identifier_to_locale</code> is needed
-- 
2.37.3