emacs: Clarify description of thread manipulating functions

It is not clear whether the term "thread" refers to the thread in the
database or to the thread currently shown in a buffer. Those two
meanings may refer to different sets of messages, e.g. when a new email
is added to the database while the buffer shows the state before the new
email arrived.

This patch replaces the term thread with the term current buffer, which
is hopefully less ambiguous.