Documentation: suggest `reset --merge` in How Merge Works section

Documentation: suggest `reset --merge` in How Merge Works section

The 'merge' manual suggests 'reset' to cancel a merge at the end
of the Merge Strategies list.  It is more logical to explain this
right before explaining how merge conflicts work, so the daunted
reader can have a way out when he or she needs it most.

While at it, make the advice more dependable and self-contained
by providing the --merge option.

Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Thomas Rast <trast@student.ethz.ch>