doc: git-pull: move <repository> and <refspec> params

doc: git-pull: move <repository> and <refspec> params

From user feedback:

- it's confusing that we use both <branch> and <refspec> to refer to the
  second argument
- one user is not clear about what `refs/heads/*:refs/remotes/origin/*`
  is meant to be an example of ("is it like a path?")

The DESCRIPTION section is also doing a lot right now: it's trying to
describe both how the <repository> and <refspec> arguments work (which
is pretty complex, as seen in the DEFAULT BEHAVIOUR section)
as well as how `git pull` calls `git fetch` and merge/rebase/etc
depending on the arguments.

Handle this by moving the description of the <repository> and <refspec>
arguments to the OPTIONS section, so that we can focus on the
merge/rebase/etc behaviour in the DESCRIPTION section, and refer folks
to the later sections for details.

Use the term "upstream" instead of 'the "remote" and "merge"
configuration for the current branch' since users are more likely to
know what an "upstream" is.

Signed-off-by: Julia Evans <julia@jvns.ca>
Signed-off-by: Junio C Hamano <gitster@pobox.com>