Better way of emphasizing S, L, and S in docs

Pull req #6972 negated a 100% intentional design decision, intended to
draw attention to the S, L, and S in "SaLt State file" to show why they
are called "SLS files". This commit reverts that change, and bolds the
S, L, and S so that this is more apparent.
Merge pull request #6975 from terminalmage/docs

Better way of emphasizing S, L, and S in docs