Clarify the behavior of leading slash in section title

[xuhdev]

See the confusion in https://github.com/editorconfig/editorconfig-core-test/issues/37#issuecomment-614380017

[florianb]

I'd stick with the .gitignore standard ans make the reference explicit:

The slash / is used as the directory separator. Separators may occur at the beginning, middle or end of the .gitignore search pattern. If there is a separator at the beginning or middle (or both) of the pattern, then the pattern is relative to the directory level of the particular .gitignore file itself. Otherwise the pattern may also match at any level below the .gitignore level.