(describe_map_tree): Insert key_heading here.

(describe_map_tree): Insert key_heading here.
New arg TITLE.
(describe_buffer_bindings): Corresponding changes.

(shadow_lookup): New function.
(describe_map_2): Call it.  SHADOW is now a list of maps.
(describe_vector): Likewise.
(describe_map): SHADOW is now a list of maps.
(describe_map_tree): Likewise.
(describe_buffer_bindings): Build suitable list to pass as SHADOW.
(Faccessible_keymaps): New arg PREFIX.  Callers changed.
(describe_map_tree): New arg PREFIX.
(Fdescribe_bindings): New arg PREFIX.
Pass to describe_buffer_bindings along with buffer.
(describe_buffer_bindings): Extract PREFIX and pass along.