Documentation: edit awkward references to `git merge-recursive`

Documentation: edit awkward references to `git merge-recursive`

A few places in the documentation referred to the "`recursive` strategy"
using the phrase "`git merge-recursive`", suggesting that it was forking
subprocesses to call a toplevel builtin.  Perhaps that was relevant to
when rebase was a shell script, but it seems like a rather indirect way
to refer to the `recursive` strategy.  Simplify the references.

Acked-by: Derrick Stolee <dstolee@microsoft.com>
Acked-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>