Try to clarify legal names

The current definition:

> The tag's content MUST be a non-whitespace character sequence NOT
> containing the current closing delimiter.

That would imply that `{{content}}` is legal, but `{{ content }}` is
not legal.

However, later tests state “Superfluous in-tag whitespace should be
ignored.” and include tests for `{{ string }}`.

This change attempts to make that clearer and consistent everywhere.


Possibly a BNF or similar grammar would be helpful?