manual generation: update help

The 'make help' lines for generating the manual did not consider ePub
correctly.  Instead of providing a limitative list for 'make manual', the
text is changed into 'all formats'.

Signed-off-by: Thomas De Schampheleire <thomas.de.schampheleire@gmail.com>
Acked-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>