scalar: reword command documentation to clarify purpose
Rephrase documentation to describe scalar as a "large repo management tool"
rather than an "opinionated management tool". The new description is
intended to more directly reflect the utility of scalar to better guide
users in preparation for scalar being built and installed as part of Git.
Signed-off-by: Victoria Dye <vdye@github.com> Acked-by: Derrick Stolee <derrickstolee@github.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>