Document veb(4)

Document veb(4)

All text is copied from other already existing sections, i.e. link flag
handling from TPMR and the rest from BIDGE.

Contrary to BRIDGE, add a synopsis for VEB such that there's a simple
overwiew, especially since veb(4) currently does not explain *how* to use
the described features.

NB: While TPMR and VEB use the same wording for link flags, their semantics
are different, i.e. both different flags and swapped polarity for those
flags.

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