Update documentation for the clock parameters

It is followed by a number of rows of the form:

    TYPE COUNT PAUSE PATH [#RGBHEX CLOCK]

    TYPE COUNT PAUSE PATH [#RGBHEX [CLOCK1 [CLOCK2]]]

  * **TYPE:** a single char indicating what type of animation segment this is:

      + `p` -- this part will play unless interrupted by the end of the boot

  * **PAUSE:** number of FRAMES to delay after this part ends

  * **PATH:** directory in which to find the frames for this part (e.g. `part0`)

  * **RGBHEX:** _(OPTIONAL)_ a background color, specified as `#RRGGBB`

  * **CLOCK:** _(OPTIONAL)_ the y-coordinate at which to draw the current time (for watches)

  * **CLOCK1, CLOCK2:** _(OPTIONAL)_ the coordinates at which to draw the current time (for watches):

      + If only `CLOCK1` is provided it is the y-coordinate of the clock and the x-coordinate

        defaults to `c`

      + If both `CLOCK1` and `CLOCK2` are provided then `CLOCK1` is the x-coordinate and `CLOCK2` is

        the y-coodinate

      + Values can be either a positive integer, a negative integer, or `c`

          - `c` -- will centre the text

          - `n` -- will position the text n pixels from the start; left edge for x-axis, bottom edge

            for y-axis

          - `-n` -- will position the text n pixels from the end; right edge for x-axis, top edge

            for y-axis

          - Examples:

              * `-24` or `c -24` will position the text 24 pixels from the top of the screen,

                centred horizontally

              * `16 c` will position the text 16 pixels from the left of the screen, centred

                vertically

              * `-32 32` will position the text such that the bottom right corner is 32 pixels above

                and 32 pixels left of the edges of the screen

There is also a special TYPE, `$SYSTEM`, that loads `/system/media/bootanimation.zip`

and plays that.

## clock_font.png

The file used to draw the time on top of the boot animation. The font format is as follows:

  * The file specifies glyphs for the ascii characters 32-127 (0x20-0x7F), both regular weight and

    bold weight.

  * The image is divided into a grid of characters

  * There are 16 columns and 6 rows

  * Each row is divided in half: regular weight glyphs on the top half, bold glyphs on the bottom

  * For a NxM image each character glyph will be N/16 pixels wide and M/(12*2) pixels high

## loading and playing frames

Each part is scanned and loaded directly from the zip archive. Within a part directory, every file