added information to the documentation of IsPrimitive

lib/oprt.gd

@@ -1783,6 +1783,7 @@ OrbitsishOperation( "IsTransitive", OrbitsishReq, false, NewProperty );
 #############################################################################
 ##
 #O IsPrimitive( <G>, <Omega>[, <gens>, <acts>][, <act>] )
+#P IsPrimitive( <G> )
 #P IsPrimitive( <xset> )
 ##
 ## <#GAPDoc Label="IsPrimitive">
@@ -1799,7 +1800,20 @@ OrbitsishOperation( "IsTransitive", OrbitsishReq, false, NewProperty );
 ## <P/>
 ## <Index>primitive</Index>
 ## An action is <E>primitive</E> if it is transitive and the action admits
-## no nontrivial block systems. See&nbsp;<Ref Sect="Block Systems"/>.
+## no nontrivial block systems. See&nbsp;<Ref Sect="Block Systems"/> for
+## the definition of block systems.
+## <P/>
+## For permutation groups, the syntax <C>IsPrimitive(<A>G</A>)</C> is also
+## permitted and tests whether the group is primitive on the points moved
+## by it, that is the group <M>\langle (2,3,4),(2,3) \rangle</M>
+## is primitive (on 3 points).
+## <P/>
+## For an explanation of the meaning of all the inputs, please refer to the
+## introduction of this chapter.
+## See&nbsp;<Ref Sect="About Group Actions"/>.
+## <P/>
+## <E>Note:</E> This operation does not tell whether a matrix group
+## preserves a direct sum of vector spaces.
 ## <Example><![CDATA[
 ## gap> IsPrimitive(g,Orbit(g,(1,2)(3,4)));
 ## true