trace: Format strings must begin/end with double quotes

Document the restriction that format strings must begin and end with
double quotes.  This is for easy parsing since we don't run cpp over
trace-events.
Signed-off-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>
Signed-off-by: Blue Swirl <blauwirbel@gmail.com>

docs/tracing.txt

@@ -74,7 +74,10 @@ Trace events should use types as follows:
 
 Format strings should reflect the types defined in the trace event.  Take
 special care to use PRId64 and PRIu64 for int64_t and uint64_t types,
-respectively.  This ensures portability between 32- and 64-bit platforms.
+respectively.  This ensures portability between 32- and 64-bit platforms.  Note
+that format strings must begin and end with double quotes.  When using
+portability macros, ensure they are preceded and followed by double quotes:
+"value %"PRIx64"".
 
 === Hints for adding new trace events ===