Make top comment section match project.

-Implement CH3 and CH4 optionally via #define
-By default only CH1 and CH2 are enabled.
-Channels 3 and 4 can be enabled by uncommenting the appropriate line
-Choose whether default of PWM pins is HIGH (0xff) or LOW (0x00)

NOTES:
-Channel 3 is on a different port (GPIOC) which will be enabled if
needed.

-Channel 4 is on the NRST pin so not usable unless you use `minichlink -d`
to disable the NRST function so the pin can be used as a GPIO

-If your board has a bypass capacitor on PD7 for a reset button the
waveforms observed on CH4 will not be nice square waves.

Uses partially remapped pins to read a rotary encoder (or any quadrature
input) using pins that should be 5V tolerant

These boards have an LED on D6

https://github.com/wuxx/nanoCH32V003

SYSTEM_CORE_CLOCK went away in favor of FUNCONF_SYSTEM_CORE_CLOCK

APB_CLOCK is only used in the i2c_slave.h and ch32v003_SPI.h files
so the references there have been maintained.

Example how to use the pin-change interrupt to monitor a cap touch sensor with about 6 bits of accuracy.

 * No more needing to decide between printf and UART in code.
 * Better debugprintf input
 * Unified code for all clock sources.
 * TINYVECTOR now saves like another 120 bytes of space.
 * None of the `Makefile`s except `bootloader` need anything extra
 * All configuration via `funconfig.h`
 * Sane defaults.
 * `SystemInit()` also initializes debugging (UART or SWIO depending on configuration)
 * Tunable timeout on debug printf.
 * Configurable UART baud rate.
 * `funconfig.h` is automatically included by `ch32v003fun.h`
 * Paves the way for other, non-003 processors.