modernize fetch/merge/pull examples

modernize fetch/merge/pull examples

The "git pull" documentation has examples which follow an outdated
style. Update the examples to use "git merge" where appropriate and
move the examples to the corresponding manpages.

Furthermore,

 - show that pull is equivalent to fetch and merge, which is still a
   frequently asked question,

 - explain the default fetch refspec.

Signed-off-by: Clemens Buchacher <redacted>
Signed-off-by: Junio C Hamano <redacted>