A couple of doc updates

Clarify the fact that temporary HV/BE params in instance start override
and do not extend the configured parameters; and change the instance
list headers from HVM_* to * since many of the parameters apply to KVM
too. Also fix a typo in the rapi documention for '/2/nodes'.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>