Clarify valid characters following opening # sequence in ATX headers

[xi2]

Hi. I'm new to CommonMark so forgive me if I've misread/misunderstood things.

In section 4.2 of the v0.22 spec it says:

The opening sequence of # characters cannot be followed directly by a non-whitespace character.

However, the rest of section 4.2 never mentions whitespace characters, only space characters. The examples seem to suggest only space characters are permitted following the opening sequence of # characters. If I'm correct, would it be clearer to change the above quoted sentence to say "non-space character" rather than "non-whitespace character"?