patch 9.1.1357: Vim incorrectly escapes tags with "[" in a help buffer

patch 9.1.1357: Vim incorrectly escapes tags with "[" in a help buffer

Problem:  Vim incorrectly escapes tags containing "[" in a help buffer
Solution: check if the buffer has the "help" filetype set, instead of
          already being a help buffer (Phạm Bình An)

fixes: #17224
closes: #17232

Co-authored-by: Christian Brabandt <cb@256bit.org>
Co-authored-by: zeertzjq <zeertzjq@outlook.com>
Signed-off-by: Phạm Bình An <phambinhanctb2004@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>