Clarify the effects of path separator

[xuhdev]

This is consistent with the rules from .gitignore: https://git-scm.com/docs/gitignore

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.

Since we intend to be consistent with .gitignore, this part of the spec has been unclear.

Fix editorconfig/editorconfig#509

[xuhdev]

And we have been unclear with slashes at the end because it didn't matter as matching directories is pointless...