Update the manpage.

Update the manpage.

 - Don't refer to the not yet existing nexus-osciprime driver.

 - Update an example sigrok-cli --scan output to match the current format.

 - Update some sections to honor that channels are now specified by
   name (not number). Example: "-P uart:rx=FTDIRX" instead of "-P uart:rx=6"
   (if channel 6 has been actually named "FTDIRX" by the user/frontend).

   This fixes bug #332.

 - SPI PD: The 'sck' pin is now called 'clk', update the manpage example.

 - Fix a typo in the -S documentation: s/-A/-P/.

 - The --time option currently only supports "ms" and "s" as units.

 - Various minor cosmetic changes.