add history entry

Author: dkarrasch

docs/src/history.md

@@ -20,6 +20,14 @@
   explicit Khatri-Rao product in memory and then multiplying with the vector; not to
   mention the memory savings. Unfortunately, similar efficiency cannot be achieved for the
   face-splitting product.
+* The construction of function-based `LinearMap`s, typed `FunctionMap`, has been rearranged.
+  Instead of the convenience constructor `LinearMap{T=Float64}(f, [fc,] M, N=M; kwargs...)`,
+  it is now recommended to use the newly exported `FunctionMap{T,iip}(f, [fc], M, N; kwargs...)`
+  constructor. Here, `iip` is either `true` or `false`, and encodes whether `f` (and `fc` if
+  present) are mutating functions. Previously, this was determined via the `Bool` keyword
+  argument `ismutating`. The re-design was made in a largely backwards-compatible manner,
+  though type inference of construction calls via `LinearMaps` is now somewhat limited.
+  This change allows for a reduction of allocated memory for non-mutating `FunctionMap`s.
 
 ## What's new in v3.8