Expose UART 1 (#1934)

* Expose UART 1 and update uart module documentation

app/platform/cpu_esp8266.h

@@ -11,7 +11,7 @@
 // Number of resources (0 if not available/not implemented)
 #define NUM_GPIO              GPIO_PIN_NUM
 #define NUM_SPI               2
-#define NUM_UART              1
+#define NUM_UART              2
 #define NUM_PWM               GPIO_PIN_NUM
 #define NUM_ADC               1
 #define NUM_CAN               0

docs/en/modules/uart.md

@@ -7,6 +7,10 @@ The [UART](https://en.wikipedia.org/wiki/Universal_asynchronous_receiver/transmi
 
 The default setup for the uart is controlled by build-time settings. The default rate is 115,200 bps. In addition, auto-baudrate detection is enabled for the first two minutes
 after platform boot. This will cause a switch to the correct baud rate once a few characters are received. Auto-baudrate detection is disabled when `uart.setup` is called.
+
+!!! important
+	Although there are two UARTs(0 and 1) available to NodeMCU, **UART 1 is not capable of receiving data and is therefore transmit only**.
+	
 ## uart.alt()
 Change UART pin assignment.
 
@@ -28,6 +32,9 @@ Sets the callback function to handle UART events.
 
 Currently only the "data" event is supported.
 
+!!! note 
+	Due to limitations of the ESP8266, only UART 0 is capable of receiving data.  
+
 #### Syntax
 `uart.on(method, [number/end_char], [function], [run_input])`
 
@@ -77,7 +84,7 @@ end, 0)
 `uart.setup(id, baud, databits, parity, stopbits[, echo])`
 
 #### Parameters
-- `id` always zero, only one uart supported
+- `id` UART id (0 or 1).
 - `baud` one of 300, 600, 1200, 2400, 4800, 9600, 19200, 31250, 38400, 57600, 74880, 115200, 230400, 256000, 460800, 921600, 1843200, 3686400
 - `databits` one of 5, 6, 7, 8
 - `parity` `uart.PARITY_NONE`, `uart.PARITY_ODD`, or `uart.PARITY_EVEN`
@@ -101,7 +108,7 @@ Returns the current configuration parameters of the UART.
 `uart.getconfig(id)`
 
 #### Parameters
-- `id` always zero, only one uart supported
+- `id` UART id (0 or 1).
 
 #### Returns
 Four values as follows:
@@ -127,7 +134,7 @@ Write string or byte to the UART.
 `uart.write(id, data1 [, data2, ...])`
 
 #### Parameters
-- `id` always 0, only one UART supported
+- `id` UART id (0 or 1).
 - `data1`... string or byte to send via UART
 
 #### Returns