Write some amount of documentation

docs/vibrato.md

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+# Vibrato
+
+Add pitch vibrato to oscillator sounds.
+
+## Basic Vibrato
+
+```forth
+"lead" s
+  60 note
+  5 vib        ( vibrato rate in Hz )
+  0.5 vibmod   ( vibrato depth )
+.
+```
+
+## Vibrato Shape
+
+Control the LFO waveform:
+
+```forth
+"pad" s
+  c4 note
+  4 vib
+  0.3 vibmod
+  0 vibshape   ( 0=sine, other values for different shapes )
+.
+```
+
+## Subtle vs Expressive
+
+```forth
+( subtle vibrato for pads )
+"pad" s 3 vib 0.1 vibmod .
+
+( expressive vibrato for leads )
+"lead" s 6 vib 0.8 vibmod .
+```
+
+## Words
+
+| Word | Stack | Description |
+|------|-------|-------------|
+| `vib` | (f --) | Set vibrato rate (Hz) |
+| `vibmod` | (f --) | Set vibrato depth |
+| `vibshape` | (f --) | Set vibrato LFO shape |