Incomplete documentation of PWM frequencies in analogWrite() reference

[per1234]

Of PWM frequencies, the analogWrite() reference page says only:

The frequency of the PWM signal on most pins is approximately 490 Hz. On the Uno and similar boards, pins 5 and 6 have a frequency of approximately 980 Hz.

This statement is not correct for all boards.

Originally reported at: arduino/Arduino#8818

[animeshsrivastava24]

@per1234 Also, if we want we can cite a reference to analogWriteResolution of
Arduino MKR