I am not sure who wrote the documents but I took a gander. Admittedly it could be improved but I don't find it so hard to understand. Basically one is redirected to the constraint command although a link within the pdf could be useful...
The lbboundary command syntax is very close to the constraint syntax, as
usually one wants the hydrodynamic boundary conditions to be shaped similarily
to the MD boundaries. Currently the shapes mentioned above are available and
their syntax exactly follows the syntax of the constraint command. For example
lbboundary wall dist 1.5 normal 1. 0. 0.
creates a planar boundary condition at distance 1.5 from the origin of the coordinate
system where the half space x > 1.5 is treated as normal LB fluid, and the other half
space is filled with boundary nodes.