Better document bearing-only measurement tangent

[dehann]

Why [2] in X[2] here:

• RoME.jl/src/factors/Bearing2D.jl

  Line 29 in f7347f4

  δθ = Manifolds.sym_rem(X[2] - atan(pl[2], pl[1]))

[dehann]

X[2] because we store measurement tangent as 2x2 matrix (Lie algebra, so(2)), so:

[0 -a
a 0]