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

In commit b0eaa4c5, 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

contrib/pgstattuple/pgstatapprox.c

@@ -89,6 +89,9 @@ statapprox_heap(Relation rel, output_type *stat)
 		/*
 		 * If the page has only visible tuples, then we can find out the free
 		 * space from the FSM and move on.
+		 *
+		 * Note: If a relation has no FSM, GetRecordedFreeSpace() will report
+		 * zero free space.  This is fine for the purposes of approximation.
 		 */
 		if (VM_ALL_VISIBLE(rel, blkno, &vmbuffer))
 		{

doc/src/sgml/pgfreespacemap.sgml

@@ -61,6 +61,8 @@
    The values stored in the free space map are not exact. They're rounded
    to precision of 1/256th of <symbol>BLCKSZ</symbol> (32 bytes with default <symbol>BLCKSZ</symbol>), and
    they're not kept fully up-to-date as tuples are inserted and updated.
+   In addition, small tables don't have a free space map, so these functions
+   will return zero even if free space is available.
   </para>
 
   <para>

doc/src/sgml/pgstattuple.sgml

@@ -527,7 +527,9 @@ approx_free_percent  | 2.09
       bit set, then it is assumed to contain no dead tuples). For such
       pages, it derives the free space value from the free space map, and
       assumes that the rest of the space on the page is taken up by live
-      tuples.
+      tuples. Small tables don't have a free space map, so in that case
+      this function will report zero free space, likewise inflating the
+      approximate tuple length.
      </para>
 
      <para>