MHCLO documentation missing `tag` description

[Shatur]

Describe the bug

Most MHCLO files have tag, but docs don't mention it. Only MHMAT mentions it:

mpfb2/docs/fileformats/mhmat.md

Line 30 in a367ecd

- tag `<tag_text>` - Categorization tag. Can appear multiple times for multiple tags

[joepal1976]

Thanks. It's somewhere between a bug and a documentation issue: MPFB actually does not support the tag key at all, it's simply ignored. Optimally it should be used for something useful instead.