doc: version: document the numeric version limits

The `VERSION` numeric fields are used to construct the `uint32_t`
`KERNELVERSION` and `APPVERSION` values, which only allocates a single
byte for each field.

The field in `app_version.h` is `APP_TWEAK`, not `APP_VERSION_TWEAK`.

Signed-off-by: Jordan Yates <jordan@embeint.com>