(!HELP NEEDED) Where is the doc for the format strings in gcc (for example, %q+D, ...)

[Qing Zhao via Gcc-patches]

Hi,

In GCC, there are many utility routines for reporting error, warning, or 
information, for example:

warning (0, "weak declaration of %q+D not supported", decl);
warning_at (stmtloc, OPT_Wmaybe_uninitialized,  "%qE may be used 
uninitialized", ptr));
inform (loc, "in a call to %qT declared with " "attribute %<%s%>", fntype, 
access_str);
error ("%qD is unavailable: %s", node, (const char *) msg);

There are format-strings inside them, “%q+D”, “%qE”, “%qT”, “%qD”, etc, where 
can I find a doc for the details of
These format-strings? Or which source files I should read to understand the 
details?

Thanks a lot for your help.

Qing