Fix the return value description of libssh2_knownhost_free().

docs/libssh2_knownhost_free.3

@@ -11,8 +11,7 @@ void libssh2_knownhost_free(LIBSSH2_KNOWNHOSTS *hosts);
 .SH DESCRIPTION
 Free a collection of known hosts.
 .SH RETURN VALUE
-Returns a handle pointer or NULL if something went wrong. The returned handle
-is used as input to all other known host related functions libssh2 provides.
+None.
 .SH AVAILABILITY
 Added in libssh2 1.2
 .SH SEE ALSO