improving doc

src/help/11-math.txt

@@ -33,3 +33,19 @@ orange dot marks the output node.
 same source passes 50 Hz at 0.97 and 2 kHz at 0.002; a 1 kHz band pass with Q=2 passes 200 Hz at
 0.05, 1 kHz at 0.95 and 4 kHz at 0.05. Each derived channel must hold exactly as many samples as
 the shared clock has time slots.
+
+<b>Why an operation sometimes does nothing</b>. A graph's <i>output</i> is the node whose value
+becomes the channel, and it is drawn as the orange dot. A new graph is built source, then operation,
+and the last node added becomes the output -- but a graph whose output is left on the source is
+valid and simply passes its input through, which is the usual reason a gain or a filter appears to
+have no effect. <i>Set as output</i> moves the dot.
+
+<b>Save refuses an incomplete graph</b>, and says why: an input that is not connected, no output
+chosen, or a <i>channel</i> node naming a signal the scope is not acquiring. Only the channels being
+acquired are offered, because a source that is not acquired reads as no data for ever -- and a graph
+that reads NaN is harder to diagnose than one that would not save.
+
+<b>The filters are real filters.</b> A math channel's samples are produced as the stream arrives,
+once per stored slot, so a low pass carries the history a filter needs. A cutoff typed in hertz is
+against the <i>stored</i> rate, which is the wire rate until a wide window makes the buffer decimate
+-- the status line's rate is that number, and the filters are rebuilt whenever it changes.