Update api.md to reflect documentation support for 5x10 displays (#70)

arduino/Arduino#5563 (comment)

Found this issue in the arduino/Arduino repository where an user had trouble finding the documentation for her unique 1 line 5x10 char LCD display in the LiquidCrystal references page, since she could not display the extended 32 characters with the default dotsize value in:

void begin(uint8_t cols, uint8_t rows, uint8_t charsize = LCD_5x8DOTS);

She had to manually dig into the source code to define the extended 5x10 font support. This commit should help users with similar boards get started easily with the LiquidCrystal library using the overloaded begin() invocation.

Author: buddywhitman

docs/api.md

@@ -51,7 +51,7 @@ Initializes the interface to the LCD screen, and specifies the dimensions (width
 #### Syntax
 
 ```
-lcd.begin(cols, rows)
+lcd.begin(cols, rows, charsize)
 ```
 
 #### Parameters
@@ -61,6 +61,8 @@ cols: the number of columns that the display has
 
 rows: the number of rows that the display has
 
+charsize (optional): the number of dots the display has per character: LCD_5x8DOTS for 5x8, LCD_5x10DOTS for 5x10. (default: LCD_5x8DOTS)
+
 ### `clear()`
 
 #### Description
@@ -421,4 +423,4 @@ void setup() {
 }
 
 void loop() {}
-```
+```