table tree types are confusingly named

[Gavin Smith]

I'm having a hard time making sense of the code dealing with @table
and @item, partly because of the naming:

table_term
table_item
table_entry

  Those containers appear in C<@table>, C<@ftable> and C<@vtable>.
  A I<table_entry> container contains an entire row of the table.
  It contains a I<table_term> container, which holds all the C<@item> and
  C<@itemx> lines.  This is followed by a I<table_item> container, which
  holds the content that is to go into the second column of the table.

I think it would be improved by renaming table_item.  It's confusing
because table_item doesn't contain @item, and "item" and "entry" mean
pretty much the same thing in English.

I'm going to change it to table_definition.