pg_total_relation_size() and CHECKPOINT

Hello,

I've detected that a result value of pg_total_relation_size() for an
actively updated table might be significantly differ from a result that
is returned after explicit CHECKPOINT command execution.

select version();

"PostgreSQL 8.2.6 on i686-pc-mingw32, compiled by GCC gcc.exe (GCC)
3.4.2 (mingw-special)"

I understand the reasons of such behavior: cache buffers must be flushed
in order to be sure that pg_total_relation_size() result will be like we
expect.

Is this right?

If so I think it would be very useful to add a description of this
peculiarity to the PG documentation.

Thanks in advance,

Zubkovsky Sergey