Opps, pin 20 was never an option. Remove from docs.

WS2812Serial.cpp

 		break;
 
 	  case 8: // Serial3
-	  case 20:
 		uart = &KINETISK_UART2;
 		divisor = BAUD2DIV3(4000000);
 		portconfig = PORT_PCR_DSE | PORT_PCR_SRE | PORT_PCR_MUX(3);

examples/BasicTest/BasicTest.ino

 
 // Usable pins:
 //   Teensy LC:   1, 4, 5, 24
-//   Teensy 3.2:  1, 5, 8, 10, 20, 31
-//   Teensy 3.5:  1, 5, 8, 10, 20, 26, 32, 33, 48
-//   Teensy 3.6:  1, 5, 8, 10, 20, 26, 32, 33
+//   Teensy 3.2:  1, 5, 8, 10, 31   (overclock to 120 MHz for pin 8)
+//   Teensy 3.5:  1, 5, 8, 10, 26, 32, 33, 48
+//   Teensy 3.6:  1, 5, 8, 10, 26, 32, 33
 
 byte drawingMemory[numled*3];         //  3 bytes per LED
 DMAMEM byte displayMemory[numled*12]; // 12 bytes per LED

examples/FastLED_Cylon/FastLED_Cylon.ino

 
 // Usable pins:
 //   Teensy LC:   1, 4, 5, 24
-//   Teensy 3.2:  1, 5, 8, 10, 20, 31
-//   Teensy 3.5:  1, 5, 8, 10, 20, 26, 32, 33, 48
-//   Teensy 3.6:  1, 5, 8, 10, 20, 26, 32, 33
+//   Teensy 3.2:  1, 5, 8, 10, 31   (overclock to 120 MHz for pin 8)
+//   Teensy 3.5:  1, 5, 8, 10, 26, 32, 33, 48
+//   Teensy 3.6:  1, 5, 8, 10, 26, 32, 33
 #define DATA_PIN 1
 
 // Define the array of leds

readme.md

 | :------ | :---------: | :--------: | :--------: | :--------: |
 | Serial1 | 1, 4, 5, 24 | 1, 5       | 1, 5, 26   | 1, 5, 26   |
 | Serial2 |             | 10, 31     | 10         | 10         |
-| Serial3 |             | 8, 20      | 8, 20      | 8, 20      |
+| Serial3 |             | 8          | 8          | 8          |
 | Serial4 |             |            | 32         | 32         |
 | Serial5 |             |            | 33         | 33         |
 | Serial6 |             |            | 48         |            |
 Serial2 & Serial3 on Teensy LC are not supported, due to lack of configurable
 oversampling needed to run at the high speed required.
 
+Serial3-Serial6 should be used only with CPU speeds 120 or 180 MHz.
+
 Serial6 on Teensy 3.6 is not currently supported, due to different hardware
 registers.