pgsql: Doc: Update the documentation for FSM behavior for smalltables.

Doc: Update the documentation for FSM behavior for small tables.

In commit b0eaa4c51b, we have avoided the creation of FSM for small tables.
So the functions that use FSM to compute the free space can return zero for
such tables.  This was previously also possible for the cases where the
vacuum has not been triggered to update FSM.

This commit updates the comments in code and documentation to reflect this
behavior.

Author: John Naylor
Reviewed-by: Amit Kapila
Discussion: https://postgr.es/m/CACPNZCtba-3m1q3A8gxA_vxg=T7gQf7gMbpR4Ciy5LntY-j+0Q@mail.gmail.com

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/29d108cdecbe918452e70041d802cc515b2d56b8

Modified Files
--------------
contrib/pgstattuple/pgstatapprox.c | 3 +++
doc/src/sgml/pgfreespacemap.sgml   | 2 ++
doc/src/sgml/pgstattuple.sgml      | 4 +++-
3 files changed, 8 insertions(+), 1 deletion(-)