Wrong documentation about @import rule for theme

[yhatt]

Documentation says:

@import must precede all other statements excepted @charset. It follows the original specification.

However, this restriction is gone by #72. @import could work correctly in anywhere of the CSS root, because we have a roll-up process for @import and @charset before convertion. It becomes that @import and @import-theme behave almost same behavior while theme conversion.

We have to update documentation to avoid confusion.