documented the new .gcsafe override pragma

doc/manual/threads.txt

@@ -49,6 +49,21 @@ To disable the GC-safety checking the ``--threadAnalysis:off`` command line
 switch can be used. This is a temporary workaround to ease the porting effort
 from old code to the new threading model.
 
+To override the compiler's gcsafety analysis a ``{.gcsafe.}`` pragma block can
+be used:
+
+.. code-block:: nim
+
+  var
+    someGlobal: string = "some string here"
+    perThread {.threadvar.}: string
+
+  proc setPerThread() =
+    {.gcsafe.}:
+      deepCopy(perThread, someGlobal)
+
+  onThreadCreation(setPerThread)
+
 
 Future directions:
 

doc/spawn.txt

@@ -92,5 +92,4 @@ restrictions / changes:
 * Every array access has to be provably within bounds. This is called
   the *bounds check*.
 * Slices are optimized so that no copy is performed. This optimization is not
-  yet performed for ordinary slices outside of a ``parallel`` section. Slices
+  yet performed for ordinary slices outside of a ``parallel`` section.
-  are also special in that they currently do not support negative indexes!