Improve comments for execExpr.c's handling of FieldStore subexpressions.

Given this code's general eagerness to use subexpressions' output variables
as temporary workspace, it's not exactly clear that it is safe for
FieldStore to tell a newval subexpression that it can write into the same
variable that is being supplied as a potential input.  Document the chain
of assumptions needed for that to be safe.

src/backend/executor/execExpr.c

@@ -1116,6 +1116,18 @@ ExecInitExprRec(Expr *node, PlanState *parent, ExprState *state,
 					 * field assignment can't be within a CASE either.  (So
 					 * saving and restoring innermost_caseval is just
 					 * paranoia, but let's do it anyway.)
+					 *
+					 * Another non-obvious point is that it's safe to use the
+					 * field's values[]/nulls[] entries as both the caseval
+					 * source and the result address for this subexpression.
+					 * That's okay only because (1) both FieldStore and
+					 * ArrayRef evaluate their arg or refexpr inputs first,
+					 * and (2) any such CaseTestExpr is directly the arg or
+					 * refexpr input.  So any read of the caseval will occur
+					 * before there's a chance to overwrite it.  Also, if
+					 * multiple entries in the newvals/fieldnums lists target
+					 * the same field, they'll effectively be applied
+					 * left-to-right which is what we want.
 					 */
 					save_innermost_caseval = state->innermost_caseval;
 					save_innermost_casenull = state->innermost_casenull;