Added comments to PWM and CPU timer sections.

Added comments to the PWM section in ECI_API.h regarding calculation of
switching frequency from PWM_PD.

	 * PWM Settings:
	 	 * 			Symmetrical PWM using
		 * 			TB_COUNT_UPDOWN
		 	 * 			Counter counts up to
			 * 			PWM_PD then back down
			 	 * 			so period is
				 * 			actually 2*PWM_PD.
				 	 * 			PWM_PD set
					 * 			at top of
					 * 			ECI_API.h
					 	 *
						 	 * 			2*PWM_PD
							 * 			=
							 * 			(150
							 * 			MHz
							 * 			/
							 * 			fsw)
							 	 * 			ex
								 * 			1:
								 * 			for
								 * 			fsw
								 * 			=
								 * 			10
								 * 			kHz,
								 * 			PWM_PD
								 * 			=
								 * 			7500
								 	 * 			ex
									 * 			2:
									 * 			for
									 * 			fsw
									 * 			=
									 * 			20
									 * 			kHz,
									 * 			PWM_PD
									 * 			=
									 * 			3750

And for the CPU Timer Setup section comments about using the EPWM1_INT
interrupt from EPWM1 for timer_isr so the controller interrupt is always
synchronized with the count=0 of EPWM1. Additional comments about how to
start and stop EPWM1 interrupts may be needed as the previous function
made by Troy for starting and stopping the timmer "StartTimer()" and
StopTimer() no longer work since they link to the CPU timer.
Changed back to single timer_isr function for rack 1 and 2 b2b.

Had previously made a duplicate of the entire timer_isr function with
"#if rk1b2b" if statements for each one selecting which rack you wish to
compile to. This reduces the reuse of code so it may be better to only
have one timer_isr for the back-to-back converters and split up any
differences with local "#if"s so as not to have to make a change twice if
both racks need the change.