Add multibyte chars at 1st line of ja help document

[takanabe]

• Vim checks firstline of help file whether it contains multibyte character or not for detecting help file's encoding.

• Currently ja help doc makes following errors:

Vim(helptags):E670: Mix of help file encodings within a language: /Users/takanabe/.vim/bundle/.neobundle/doc/eregex.jax

[othree]

Thanks~