README.md: improve sys/base/seq documentation
README.md: improve sys/base/seq documentation

- Consistently use the term "radix point". The term "decimal point"
  makes no sense in a command that supports any output base.
  https://en.wikipedia.org/wiki/Radix_point

- Complete and improve documentation of differences with BSD 'seq'
  and GNU 'seq'.

modernish/modernish