Fix keypad EventQueue len(); improve doc

[dhalbert]

• Fixes keypad.EventQueue.__len__() is returning wrong value #10183.
• Improves documentation, as suggested in https://forums.adafruit.com/viewtopic.php?t=217476.

Tested on a CPB:

import keypad
import board
import time

keys = keypad.Keys((board.BUTTON_A,), value_when_pressed=True, pull=True)

while True:
    print("wait 5 seconds")
    time.sleep(5)
    print(f"{len(keys.events)=}")
    count = 0
    while (keys.events.get()):
        count+= 1
    print(f"{count=}")

[Neradoc]

All good, thanks 👍