docs/manual: add documentation on Config.in.host

+ rearrange the structure a bit by adding a new paragraph for the Config
files

Signed-off-by: Jan Heylen <heyleke@gmail.com>
[ThomasDS:
  - clarify commit title
  - add list continuations to fix indentation of Config.in.host details]
Signed-off-by: Thomas De Schampheleire <thomas.de.schampheleire@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>