Documentation: teach stash/pop workflow instead of stash/apply

Documentation: teach stash/pop workflow instead of stash/apply

Recent discussion on the list showed some comments in favour of a
stash/pop workflow:

  http://marc.info/?l=git&m=124234911423358&w=2
  http://marc.info/?l=git&m=124235348327711&w=2

Change the stash documentation and examples to document pop in its own
right (and apply in terms of pop), and use stash/pop in the examples.

Signed-off-by: Thomas Rast <trast@student.ethz.ch>
Signed-off-by: Junio C Hamano <gitster@pobox.com>