adapt docstring for CG

Author: ranocha

src/cg.jl

@@ -185,7 +185,11 @@ cg(A, b; kwargs...) = cg!(zerox(A, b), A, b; initially_zero = true, kwargs...)
   residual vector;
 - `Pl = Identity()`: left preconditioner of the method. Should be symmetric,
   positive-definite like `A`;
-- `tol::Real = sqrt(eps(real(eltype(b))))`: tolerance for stopping condition `|r_k| / |r_0| ≤ tol`;
+- `abstol::Real = zero(real(eltype(b)))`,
+  `reltol::Real = sqrt(eps(real(eltype(b))))`: absolute and relative
+  tolerance for the stopping condition
+  `|r_k| / |r_0| ≤ max(reltol * resnorm, abstol)`, where `r_k = A * x_k - b`
+  is the residual in the `k`th iteration;
 - `maxiter::Int = size(A,2)`: maximum number of iterations;
 - `verbose::Bool = false`: print method information;
 - `log::Bool = false`: keep track of the residual norm in each iteration.