Merge branch 'kellertuer/refactor-initial-guess' of github.com:JuliaManifolds/Manopt.jl into kellertuer/refactor-initial-guess

Author: kellertuer

docs/src/plans/stepsize.md

@@ -32,6 +32,14 @@ WolfePowellLinesearch
 WolfePowellBinaryLinesearch
 ```
 
+Additionally, initial stepsize guesses are handled by subtypes of `AbstractInitialLinesearchGuess`:
+
+```@docs
+ConstantInitialGuess
+ArmijoInitialGuess
+HagerZhangInitialGuess
+```
+
 Some step sizes use [`max_stepsize`](@ref) function as a rough upper estimate for the trust region size.
 It is by default equal to injectivity radius of the exponential map but in some cases a different value is used.
 For the `FixedRankMatrices` manifold an estimate from Manopt is used.
@@ -58,6 +66,13 @@ Then the following state can be used to wrap these locally
 StepsizeState
 ```
 
+The Hager-Zhang initial guess uses two helper functions to determine initial stepsize in the first iteration on manifolds which have "expected minimizer", for example the zero point on the Euclidean manifold:
+
+```@docs
+default_vector_norm
+default_point_distance
+```
+
 ## Literature
 
 ```@bibliography