So, I was customizing the `speedbar' group, and I happened to notice
that the entries for `semantic-sb-button-format-tag-function' and
`semantic-sb-info-format-tag-function' offered extremely little in the
way of documentation, and while the docstrings for those variables could
do a better job of describing the contexts to which they are relevant,
the main difficulty is in knowing what the various choices mean.

A quick look in the vicinity of the functions' definitions revealed the
variable `semantic-format-tag-custom-list', which turns out to be where
the :type for those two settings comes from.  It is defined as follows: