docs: add docstring for new BVProblem constructor

Author: AayushSabharwal

src/problems/bvproblem.jl

@@ -1,3 +1,48 @@
+"""
+```julia
+SciMLBase.BVProblem{iip}(sys::AbstractSystem, u0map, tspan,
+                         parammap = DiffEqBase.NullParameters();
+                         constraints = nothing, guesses = nothing,
+                         version = nothing, tgrad = false,
+                         jac = true, sparse = true,
+                         simplify = false,
+                         kwargs...) where {iip}
+```
+
+Create a boundary value problem from the [`System`](@ref). 
+
+`u0map` is used to specify fixed initial values for the states. Every variable 
+must have either an initial guess supplied using `guesses` or a fixed initial 
+value specified using `u0map`.
+
+Boundary value conditions are supplied to Systems in the form of a list of constraints.
+These equations  should specify values that state variables should take at specific points,
+as in `x(0.5) ~ 1`). More general constraints that  should hold over the entire solution,
+such as `x(t)^2 + y(t)^2`, should be  specified as one of the equations used to build the
+`System`.
+
+If a `System` without `constraints` is specified, it will be treated as an initial value problem. 
+
+```julia
+    @parameters g t_c = 0.5
+    @variables x(..) y(t) λ(t)
+    eqs = [D(D(x(t))) ~ λ * x(t)
+           D(D(y)) ~ λ * y - g
+           x(t)^2 + y^2 ~ 1]
+    cstr = [x(0.5) ~ 1]
+    @mtkbuild pend = System(eqs, t; constraints = cstrs)
+
+    tspan = (0.0, 1.5)
+    u0map = [x(t) => 0.6, y => 0.8]
+    parammap = [g => 1]
+    guesses = [λ => 1]
+
+    bvp = SciMLBase.BVProblem{true, SciMLBase.AutoSpecialize}(pend, u0map, tspan, parammap; guesses, check_length = false)
+```
+
+If the `System` has algebraic equations, like `x(t)^2 + y(t)^2`, the resulting 
+`BVProblem` must be solved using BVDAE solvers, such as Ascher.
+"""
 @fallback_iip_specialize function SciMLBase.BVProblem{iip, spec}(
         sys::System, u0map, tspan, parammap = SciMLBase.NullParameters();
         check_compatibility = true, cse = true, checkbounds = false, eval_expression = false,