Documentation: simplify How Merge Works

Documentation: simplify How Merge Works

The user most likely does not care about the exact order of
operations because he cannot see it happening anyway.  Instead,
try to explain what it means to merge two commits into a single
tree.

While at it:

 - Change the heading to TRUE MERGE.  The entire manual page is
   about how merges work.

 - Document MERGE_HEAD.  It is a useful feature, since it makes
   the parents of the intended merge commit easier to refer to.

 - Do not assume commits named on the 'git merge' command line come
   from another repository.  For simplicity, the discussion of
   conflicts still does assume that there is only one and it is a
   branch head.

 - Do not start list items with `code`.  Otherwise, a toolchain bug
   produces a line break in the generated nroff, resulting in odd
   extra space.

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