Document scoring functionality.

Document scoring functionality.
(gametree-score-regexp): Add optional plus sign.
(gametree-score-opener): Replace `:' with `=' as the former
conflicts with gametree-half-ply-regexp.
(gametree-transpose-following-leaves): Add.
(gametree-insert-new-leaf): Make modifying commands barf in read-only buffers.
(gametree-break-line-here): Add a call to
`gametree-transpose-following-leaves'.  This maintains the
necessary invariant that on each level all leaf children precede
all nonleaf children.  This has always been implied, but left to
the user, and, unfortunately, undocumented.