Update documentation

  * fix links to the User's Guide in ghc and ghci --help messages
  * fix default stack size info in RTS help message

driver/ghc-usage.txt

@@ -73,7 +73,7 @@ Given the above, here are some TYPICAL invocations of $$:
 The User's Guide has more information about GHC's *many* options.  An
 online copy can be found here:
 
-   http://haskell.org/haskellwiki/GHC
+   http://www.haskell.org/ghc/docs/latest/html/users_guide/
 
 If you *really* want to see every option, then you can pass
 '--show-options' to the compiler.

driver/ghci-usage.txt

@@ -21,4 +21,4 @@ GHC does.  Some of the options that are commonly used are:
 Full details can be found in the User's Guide, an online copy of which
 can be found here:
 
-    http://haskell.org/haskellwiki/GHC
+    http://www.haskell.org/ghc/docs/latest/html/users_guide/

rts/RtsFlags.c

@@ -241,7 +241,8 @@ usage_text[] = {
 "  -?       Prints this message and exits; the program is not executed",
 "  --info   Print information about the RTS used by this program",
 "",
-"  -K<size> Sets the maximum stack size (default 8M)  Egs: -K32k   -K512k",
+"  -K<size>  Sets the maximum stack size (default: 80% of the heap)",
+"            Egs: -K32k -K512k -K8M",
 "  -ki<size> Sets the initial thread stack size (default 1k)  Egs: -ki4k -ki2m",
 "  -kc<size> Sets the stack chunk size (default 32k)",
 "  -kb<size> Sets the stack chunk buffer size (default 1k)",