merge-strategies.txt: update wording for the resolve strategy

merge-strategies.txt: update wording for the resolve strategy

It is probably helpful to cover the default merge strategy first, so
move the text for the resolve strategy to later in the document.

Further, the wording for "resolve" claimed that it was "considered
generally safe and fast", which might imply in some readers minds that
the same is not true of other strategies.  Rather than adding this text
to all the strategies, just remove it from this one.

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>