TH: one style fix and more docstrings

We were missing many docstrings in THH.hs, so let's add at least some
of them, and fix some unquoted '/'. Additionally one style change has
been done.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Agata Murawska <agatamurawska@google.com>