offlineimap.conf: normalize style and format
offlineimap.conf: normalize style and format

- improve style and reading for the eyes
- improve wording
- add minor comments
- remove on duplicate option
- give location of each +1 section

Signed-off-by: Nicolas Sebrecht <347709267ddd1958560c4c317ea3fd613f10a719@laposte.net>

frioux/offlineimap,frioux/offlineimap