Leave warning note about TimePosition.frame not being monotonic

Signed-off-by: falkTX <falktx@falktx.com>

distrho/DistrhoPlugin.hpp

@@ -750,6 +750,8 @@ struct TimePosition {
 
    /**
       Current host transport position in frames.
+      @note This value is not always monotonic,
+            with some plugin hosts assigning it based on a source that can accumulate rounding errors.
     */
     uint64_t frame;