DOC: simplify bind option "interface" explanation

DOC: simplify bind option "interface" explanation

The current documentation of the bind option "interface" can be misleading
(as seen on the ML recently).

This patch tries to address misunderstandings by :

  - avoiding the words listen or bind in the behavior description, using
    "restrict to interface" instead

  - using a different sentence construction (partially stolen from
    "man 7 socket": SO_BINDTODEVICE)

  - "defragmentation": moving behavior related explanations to the beginning
    and restrictions, use-cases and requirements to the end.