Write some amount of documentation

docs/mod_fx.md

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+# Modulation Effects
+
+Phaser, flanger, and chorus effects for movement and width.
+
+## Phaser
+
+```forth
+"pad" s
+  1 phaser           ( rate in Hz )
+  0.5 phaserdepth    ( depth )
+  0.5 phasersweep    ( sweep range )
+  1000 phasercenter  ( center frequency )
+.
+```
+
+## Flanger
+
+```forth
+"guitar" s
+  0.5 flanger           ( rate )
+  0.5 flangerdepth      ( depth )
+  0.5 flangerfeedback   ( feedback for metallic sound )
+.
+```
+
+## Chorus
+
+```forth
+"keys" s
+  1 chorus        ( rate )
+  0.5 chorusdepth ( depth )
+  0.02 chorusdelay ( base delay time )
+.
+```
+
+## Combining Effects
+
+```forth
+"pad" s
+  c4 note
+  0.3 phaserdepth
+  0.5 phaser
+  0.2 chorus
+  0.3 chorusdepth
+.
+```
+
+## Words
+
+| Word | Stack | Description |
+|------|-------|-------------|
+| `phaser` | (f --) | Set phaser rate |
+| `phaserdepth` | (f --) | Set phaser depth |
+| `phasersweep` | (f --) | Set phaser sweep |
+| `phasercenter` | (f --) | Set phaser center frequency |
+| `flanger` | (f --) | Set flanger rate |
+| `flangerdepth` | (f --) | Set flanger depth |
+| `flangerfeedback` | (f --) | Set flanger feedback |
+| `chorus` | (f --) | Set chorus rate |
+| `chorusdepth` | (f --) | Set chorus depth |
+| `chorusdelay` | (f --) | Set chorus delay |