fixup! Document the semantics of copyto! and add pagelock!

Author: vchuravy

src/KernelAbstractions.jl

@@ -111,14 +111,14 @@ macro Const end
     copyto!(::Backend, dest::AbstractArray, src::AbstractArray)
 
 Perform a `copyto!` operation that is execution ordered with respect to the backend.
+For most uses `Base.copyto!` provides a fully synchronous interface.
 
 !!! note
     On some backends it may be necessary to first call [`pagelock!`](@ref) on host memory,
     to enable fully asynchronous behaviour w.r.t to the host.
 
 !!! warning
-    While this function is always asynchronous w.r.t. to the device, it may be synchronous w.r.t to the host.
-    Additionally if the function is asynchronous w.r.t to the host, the user is required to gurantuee, the lifetime
+    If the function is asynchronous w.r.t to the host, the user is required to gurantuee, the lifetime
     of the host buffer. Otherwise the user may cause a use-after-free, because the GC was able to prove that the host
     buffer can be freed.