Confusion on array definition at Arduino website
1-) Isn't int mySensVals[6] = {2, 4, -8, 3, 2}; wrong? I count 5 elements.


No, not wrong at all. There are 6 elements (as the definition says), and the first five are set to 2, 4, -8, 3 and 2. The sixth is undefined.


  2-) Isn't char message[6] = "hello"; is wrong as well? I count 5 elements.


No. There are 6 there. To quote your quote:


  Note that when declaring an array of type char, one more element than your initialization is required, to hold the required null character.


So "hello" is actually "hello\0".