Clarify analog output doc for pwm limits (esp8266#6051)

* Update reference.rst

Clarify analogWrite and PWM limitations.

* Update reference.rst

Author: devyte

doc/reference.rst

@@ -70,7 +70,13 @@ equal to 1023 by default. PWM range may be changed by calling
 ``analogWriteRange(new_range)``.
 
 PWM frequency is 1kHz by default. Call
-``analogWriteFreq(new_frequency)`` to change the frequency.
+``analogWriteFreq(new_frequency)`` to change the frequency. Valid values 
+are from 100Hz up to 40000Hz.
+
+The ESP doesn't have hardware PWM, so the implementation is by software. 
+With one PWM output at 40KHz, the CPU is already rather loaded. The more 
+PWM outputs used, and the higher their frequency, the closer you get to 
+the CPU limits, and the less CPU cycles are available for sketch execution. 
 
 Timing and delays
 -----------------