Retainer prof: immutable arrays are not retainers, and fix the docs

docs/users_guide/profiling.xml

@@ -884,8 +884,8 @@ MAIN            MAIN                    102           0    0.0    0.0   100.0  1
       by defining what we mean by a retainer:</para>
 
       <blockquote>
-	<para>A retainer is either the system stack, or an unevaluated
-	closure (thunk).</para>
+        <para>A retainer is either the system stack, an unevaluated
+        closure (thunk), or an explicitly mutable object.</para>
       </blockquote>
 
       <para>In particular, constructors are <emphasis>not</emphasis>

rts/RetainerProfile.c

@@ -1013,8 +1013,6 @@ isRetainer( StgClosure *c )
     case MUT_VAR_DIRTY:
     case MUT_ARR_PTRS_CLEAN:
     case MUT_ARR_PTRS_DIRTY:
-    case MUT_ARR_PTRS_FROZEN:
-    case MUT_ARR_PTRS_FROZEN0:
 
 	// thunks are retainers.
     case THUNK:
@@ -1071,6 +1069,9 @@ isRetainer( StgClosure *c )
     case ARR_WORDS:
 	// STM
     case TREC_CHUNK:
+        // immutable arrays
+    case MUT_ARR_PTRS_FROZEN:
+    case MUT_ARR_PTRS_FROZEN0:
 	return rtsFalse;
 
 	//