BLEStringCharacteristic is undocumented

[per1234]

Moved from arduino/reference-en#965 by @cattledogGH

A BLEStringCharacteristic type was added to the library: #5

But that type is not documented in the library reference:

ArduinoBLE/docs/api.md

Lines 2664 to 2692 in dff9e41

	## BLECharacteristic Class
	Used to enable the characteristics board offers in a service or interact with characteristics a remote board provides.
	### `BLECharacteristic()`
	Create a new Bluetooth® Low Energy characteristic.
	#### Syntax
	```
	BLECharacteristic(uuid, properties, value, valueSize)
	BLECharacteristic(uuid, properties, stringValue)
	BLEBoolCharacteristic(uuid, properties)
	BLEBooleanCharacteristic(uuid, properties)
	BLECharCharacteristic(uuid, properties)
	BLEUnsignedCharCharacteristic(uuid, properties)
	BLEByteCharacteristic(uuid, properties)
	BLEShortCharacteristic(uuid, properties)
	BLEUnsignedShortCharacteristic(uuid, properties)
	BLEWordCharacteristic(uuid, properties)
	BLEIntCharacteristic(uuid, properties)
	BLEUnsignedIntCharacteristic(uuid, properties)
	BLELongCharacteristic(uuid, properties)
	BLEUnsignedLongCharacteristic(uuid, properties)
	BLEFloatCharacteristic(uuid, properties)
	BLEDoubleCharacteristic(uuid, properties)
	```

The full public API of the library must be documented.