cleanup ARC documentation (#15100)

doc/advopt.txt

@@ -146,8 +146,6 @@ Advanced options:
   --legacy:$2
                             enable obsolete/legacy language feature
   --useVersion:1.0          emulate Nim version X of the Nim compiler
-  --newruntime              use an alternative runtime that uses destructors
-                            and that uses a shared heap via -d:useMalloc
   --profiler:on|off         enable profiling; requires `import nimprof`, and
                             works better with `--stackTrace:on`
                             see also https://nim-lang.github.io/Nim/estp.html

doc/destructors.rst

@@ -534,48 +534,6 @@ indirections:
     useItAgain(v)
 
 
-
-Owned refs
-==========
-
-**Note**: The ``owned`` type constructor is only available with
-the ``--newruntime`` compiler switch and is experimental.
-
-
-Let ``W`` be an ``owned ref`` type. Conceptually its hooks look like:
-
-.. code-block:: nim
-
-  proc `=destroy`(x: var W) =
-    if x != nil:
-      assert x.refcount == 0, "dangling unowned pointers exist!"
-      `=destroy`(x[])
-
-  proc `=`(x: var W; y: W) {.error: "owned refs can only be moved".}
-
-  proc `=sink`(x: var W; y: W) =
-    `=destroy`(x)
-    bitwiseCopy x, y # raw pointer copy
-
-
-Let ``U`` be an unowned ``ref`` type. Conceptually its hooks look like:
-
-.. code-block:: nim
-
-  proc `=destroy`(x: var U) =
-    if x != nil:
-      dec x.refcount
-
-  proc `=`(x: var U; y: U) =
-    # Note: No need to check for self-assignments here.
-    if y != nil: inc y.refcount
-    if x != nil: dec x.refcount
-    bitwiseCopy x, y # raw pointer copy
-
-  proc `=sink`(x: var U, y: U) {.error.}
-  # Note: Moves are not available.
-
-
 Hook lifting
 ============