Doc: de-document unimplemented geometric operators.

In commit 791090bd, I made an effort to fill in documentation
for all geometric operators listed in pg_operator.  However,
it now appears that at least some of the omissions may have been
intentional, because some of those operator entries point at
unimplemented stub functions.  Remove those from the docs again.

(In HEAD, poly_distance stays, because c5c192d7b just added an
implementation for it.)

Per complaint from Anton Voloshin.

Discussion: https://postgr.es/m/3426566.1638832718@sss.pgh.pa.us

doc/src/sgml/func.sgml

@@ -10641,7 +10641,7 @@ CREATE TYPE rainbow AS ENUM ('red', 'orange', 'yellow', 'green', 'blue', 'purple
        </para>
        <para>
         Computes the center point.
-        Available for <type>box</type>, <type>lseg</type>, <type>path</type>,
+        Available for <type>box</type>, <type>lseg</type>,
         <type>polygon</type>, <type>circle</type>.
        </para>
        <para>
@@ -10707,8 +10707,6 @@ CREATE TYPE rainbow AS ENUM ('red', 'orange', 'yellow', 'green', 'blue', 'purple
         (<type>point</type>, <type>line</type>),
         (<type>lseg</type>, <type>box</type>),
         (<type>lseg</type>, <type>lseg</type>),
-        (<type>lseg</type>, <type>line</type>),
-        (<type>line</type>, <type>box</type>),
         (<type>line</type>, <type>lseg</type>).
        </para>
        <para>
@@ -10724,11 +10722,11 @@ CREATE TYPE rainbow AS ENUM ('red', 'orange', 'yellow', 'green', 'blue', 'purple
        </para>
        <para>
         Computes the distance between the objects.
-        Available for all seven geometric types, for all combinations
+        Available for all geometric types except <type>polygon</type>,
+        for all combinations
         of <type>point</type> with another geometric type, and for
         these additional pairs of types:
         (<type>box</type>, <type>lseg</type>),
-        (<type>box</type>, <type>line</type>),
         (<type>lseg</type>, <type>line</type>),
         (<type>polygon</type>, <type>circle</type>)
         (and the commutator cases).