[Groff] text variants

[Dave Kemper]

What's the purpose of the \[tno], \[t+-], \[tmu], and \[tdi] characters?

groff_char(7) documents these characters as "text variants" of,
respectively, \[no], \[+-], \[mu], and \[di], but provides no hints
about when one might want to use the text variant characters and when
the traditional versions.  The groff info pages don't mention text
variants at all.  This seems to be an oversight in the documentation.