Rephrase instructional code

- Now, instead of explicitly saying "you only need to do this once", I say for
  each command when you need to do it, using words like "each" and "the first
  time"

- More explanation of the Debian/Ubuntu install problem

- Remove needless warning for people using nonstandard shells.  (The
  instructions work correctly for bash, and with slight modification for zsh
  as well.  This covers all reasonable users.)