Improve comments about USE_VALGRIND in pg_config_manual.h.
These comments left the impression that USE_VALGRIND isn't really
essential for valgrind testing.  But that's wrong, as I learned
the hard way today.
Discussion:
* enables detection of buffer accesses that take place without holding a
* buffer pin (or without holding a buffer lock in the case of index access
* methods that superimpose their own custom client requests on top of the
- * generic bufmgr.c requests). See also src/tools/valgrind.supp.
+ * generic bufmgr.c requests).
*
* "make installcheck" is significantly slower under Valgrind. The client
* requests fall in hot code paths, so USE_VALGRIND slows execution by a few
* percentage points even when not run under Valgrind.
*
+ * Do not try to test the server under Valgrind without having built the
+ * server with USE_VALGRIND; else you will get false positives from sinval
+ * messaging (see comments in AddCatcacheInvalidationMessage). It's also
+ * important to use the suppression file src/tools/valgrind.supp to
+ * exclude other known false positives.
+ *
* You should normally use MEMORY_CONTEXT_CHECKING with USE_VALGRIND;
* instrumentation of repalloc() is inferior without it.
*/
