`BLEStringCharacteristic` is undocumented

[per1234]

Moved from https://github.com/arduino/reference-en/issues/965 by @cattledogGH

A BLEStringCharacteristic type was added to the library: https://github.com/arduino-libraries/ArduinoBLE/pull/5

But that type is not documented in the library reference:

https://github.com/arduino-libraries/ArduinoBLE/blob/dff9e414abc20df491b46d0b38b360eb1a3507fb/docs/api.md?plain=1#L2664-L2692

The full public API of the library must be documented.