pgstattuple documentation clarification

Recently a client was confused because there was a substantial
difference between the reported table_len of a table and the sum of the
corresponding tuple_len, dead_tuple_len and free_space. The docs are
fairly silent on this point, and I agree that in the absence of
explanation it is confusing, so I propose that we add a clarification
note along the lines of:

The table_len will always be greater than the sum of the tuple_len,
dead_tuple_len and free_space. The difference is accounted for by
page overhead and space that is not free but cannot be attributed to
any particular tuple.

Or perhaps we should be more explicit and refer to the item pointers on
the page.

Thoughts?

cheers

andrew