Use consistent naming for contents of the section

[mipo256]

When reviewing the following PR, it became clear that we use both "rules" and "pairs" sometimes in the spec to refer to the contents of the section.

I think we need to unify the spec and use consistent naming in all places

Reopened from: editorconfig/editorconfig#541

[mipo256]

We have the term "rules" utilized only in two places in the spec, both are added pretty recently. In other places the term "pairs" used overwhelmingly.

I suppose that we should change the "rules" --> "pairs". That would make sense.

What are your thoughts on this: @xuhdev @cxw42 @florianb

[xuhdev]

I agree we should avoid using the term "rules". I checked out the usage of rules in the spec; I believe they are better rephrased rather than replaced with pairs, since they refer to more than pairs. I'll make a PR.

[florianb]

I agree - but i also think we should find a good word for the logical (semantic) meaning of the pairs, like f.e. directive.

I think we don't have to agree on this yet, just for a future improvement. Since i think pair describes only on a part of the grammar and makes it less easy to comprehend what is meant with "Supported Pairs".

[cxw42]

I agree with continuing to use "pairs" for lines with an = (key-value pairs)