add a few docstrings (#613)

for `(l|r)(diffbc|dirichlet|neumann)`,  `ivp`, `bvp` and `periodic`

Author: putianyi889

src/Operators/functionals/Evaluation.jl

@@ -134,19 +134,72 @@ end
 
 
 evaluate(d::Domain,x) = Evaluation(d,x)
+
+"""
+    ldiffbc(d::Domain, k)
+
+The boundary condition of the `k`-th order derivative on the left endpoint of `d`. See also [`rdiffbc`](@ref), [`ldirichlet`](@ref) and [`lneumann`](@ref).
+"""
 ldiffbc(d,k) = Evaluation(d,leftendpoint,k)
+
+"""
+    rdiffbc(d::Domain, k)
+
+The boundary condition of the `k`-th order derivative on the right endpoint of `d`. See also [`ldiffbc`](@ref), [`rdirichlet`](@ref) and [`rneumann`](@ref).
+"""
 rdiffbc(d,k) = Evaluation(d,rightendpoint,k)
 
+"""
+    ldirichlet(d::Domain) = ldiffbc(d, 0)
+
+The dirichlet boundary condition on the left endpoint of `d`. See also [`rdirichlet`](@ref) and [`ldiffbc`](@ref).
+"""
 ldirichlet(d) = ldiffbc(d,0)
+
+"""
+    rdirichlet(d::Domain) = rdiffbc(d, 0)
+
+The dirichlet boundary condition on the right endpoint of `d`. See also [`ldirichlet`](@ref) and [`rdiffbc`](@ref).
+"""
 rdirichlet(d) = rdiffbc(d,0)
+
+"""
+    lneumann(d::Domain) = ldiffbc(d, 1)
+
+The neumann boundary condition on the left endpoint of `d`. See also [`rneumann`](@ref) and [`ldiffbc`](@ref).  
+"""
 lneumann(d) = ldiffbc(d,1)
+
+"""
+    rneumann(d::Domain) = rdiffbc(d, 1)
+
+The neumann boundary condition on the right endpoint of `d`. See also [`lneumann`](@ref) and [`rdiffbc`](@ref).  
+"""
 rneumann(d) = rdiffbc(d,1)
 
+"""
+    ivp(d::Domain, k) = [ldiffbc(d,i) for i=0:k-1]
+    ivp(d) = ivp(d,2)
 
+The conditions for the `k`-th order initial value problem. See also [`ldiffbc`](@ref), [`bvp`](@ref) and [`periodic`](@ref).
+"""
 ivp(d,k) = [ldiffbc(d,i) for i=0:k-1]
+
+"""
+    bvp(d::Domain, k) = vcat([ldiffbc(d,i) for i=0:div(k,2)-1],
+                        [rdiffbc(d,i) for i=0:div(k,2)-1])
+    bvp(d) = bvp(d,2)
+
+The conditions for the `k`-th order boundary value problem. See also [`ldiffbc`](@ref), [`rdiffbc`](@ref), [`ivp`](@ref) and [`periodic`](@ref).
+"""
 bvp(d,k) = vcat([ldiffbc(d,i) for i=0:div(k,2)-1],
                 [rdiffbc(d,i) for i=0:div(k,2)-1])
 
+"""
+    periodic(d::Domain,k) = [ldiffbc(d,i) - rdiffbc(d,i) for i=0:k]
+
+The conditions for the `k`-th order periodic problem. See also [`ldiffbc`](@ref), [`rdiffbc`](@ref), [`ivp`](@ref) and [`bvp`](@ref)
+"""
 periodic(d,k) = [ldiffbc(d,i) - rdiffbc(d,i) for i=0:k]
 
 # shorthand for second order