Add explanation of symbols used to represent whitespace characters in spec

The spec seems to use a faint middle dot character to represent a blank space, but this isn’t explained anywhere.

There is a sentence about the use of right arrows to mark tabs, in the About this document section.

I would expose a separate section for a legend, if you will, that makes the symbol use more salient (the sentence about Tabs is sort of buried at the end of that section).

Re: tabs, I had no idea that those arrows represented tabs at first. I don’t know what’s most common, but the right arrow is not listed on the Wikipedia treatment of visual representation of the tab character.

+1 for having some explicit visual legend in the specs.

It is quite common to use arrows for tabs. Even the good old Microsoft Word is using middle dots for spaces and arrows for tabs if you check the “show all characters”. If you use an editor like Notepad++, it’s doing the same.