docs: help pages for the time window, navigation, pause and trigger

src/help/05-window.txt

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+<b>Time window</b>
+
+The window is how much time the plot shows. Every row shares one window: the figure has a
+single x link group, so the rows cannot drift apart. The trigger's event span follows it, and
+the pre-trigger share is a share of <i>this</i> window.
+
+<b>How to reach it</b>
+
+The header bar's window button opens a popover: type a number, pick a unit under it, or step a
+decade with the arrows. The button's own label is a reading of the current window in a natural
+unit, width-locked so a change of window cannot widen the header bar.
+
+<b>Worth knowing</b>
+
+• A window too long to store at the wire rate is <i>decimated, not refused</i>. The shared clock
+  takes a power-of-two stride, and each group of samples is <b>averaged</b> rather than strided,
+  so a tone above the stored Nyquist is attenuated instead of aliasing to a false low frequency.
+• The stamp stored for a group is the group's <i>mean</i>, which is where the averaging puts the
+  value. Stamping at the end of a group would shift every trace half a stored sample late.
+• Moving the window across a decimation boundary rebuilds the buffer and drops the history.
+  Walking it inside one decade is free.
+• The trigger sees the <i>stored</i> (filtered) signal, not the wire signal, because an edge
+  found on a sample that is not in the buffer could not be cut out of it.
+
+<b>Example</b>
+
+1 MS/s with 200 MB of buffer. A 2 s window stores every sample (<tt>time x1</tt>). Widen it to
+600 s and the stride becomes <tt>x16</tt>: 625 S/s stored, anti-aliased at 312.5 Hz. A 2 kHz
+sine seen through that window reads <tt>y=[-0.059, 0.079]</tt> — attenuated to about 6%, not
+folded down to a full-amplitude 125 Hz one.