fixes #13881

* fixes #13881
* documented changed requirements for system.onThreadDestruction
* destructors.rst: update the documentation

lib/system.nim

@@ -692,7 +692,7 @@ proc len*(x: string): int {.magic: "LengthStr", noSideEffect.}
 proc len*(x: cstring): int {.magic: "LengthStr", noSideEffect.}
   ## Returns the length of a compatible string. This is sometimes
   ## an O(n) operation.
-  ## 
+  ##
   ## **Note:** On the JS backend this currently counts UTF-16 code points
   ## instead of bytes at runtime (not at compile time). For now, if you
   ## need the byte length of the UTF-8 encoding, convert to string with
@@ -2093,7 +2093,7 @@ when not defined(js):
 
   when hasAlloc:
     when not defined(gcRegions) and not usesDestructors:
-      proc initGC() {.gcsafe.}
+      proc initGC() {.gcsafe, raises: [].}
 
     proc initStackBottom() {.inline, compilerproc.} =
       # WARNING: This is very fragile! An array size of 8 does not work on my