Hi Stefan and Eli!

As a part of my investigation into transposing things using tree-sitter
I promised to also introduce some improvements to both the code and
documentation, as the latter is a little sparse.  I'll add some patches
while I work on the tree-sitter stuff.

This commit simplifies the transpose-chars.  The bug described in the
commit history is no longer reproducible, so I think its complicated
nature can be removed.  What do you think?

Theo