Undocumented predicates

The following predicates are exported, but not or incorrectly documented.

 expr(Arg1, Arg2)

 expr(Arg1, Arg2, Arg3)

 expr(Arg1, Arg2, Arg3, Arg4)

 expr(Arg1, Arg2, Arg3, Arg4, Arg5)

 expr(Arg1, Arg2, Arg3, Arg4, Arg5, Arg6)