Small note to add to docs

[claresinger]

Doing something like

prior1 = constrained_gaussian("prior1", 0.5e-3, 1e-3, 0.0, 1e-2)

yields

WARNING: using Distributions.cf in module Main conflicts with an existing identifier.
ERROR: LoadError: DomainError with `prior_α`: Target std 0.001 puts μ - σ too close to lower bound 0.0:

Would be nice to add to the docs that μ and σ can't be too close together or it'll throw an error.