Improve comment (explains why previous commit is correct)

src/fellow_cache.c

@@ -3219,6 +3219,12 @@ fellow_cache_lru_work(struct worker *wrk, struct fellow_cache_lru *lru)
 		WRONG("fcs state in lru");
 	}
 
+	/*
+	 * for a break from the foreach, fcs != NULL
+	 *
+	 * so for fcs == NULL, we know we have iterated over
+	 * all of the LRU
+	 */
 	if (fcs == NULL) {
 		AZ(pthread_mutex_unlock(&lru->lru_mtx));
 //		VSLb(wrk->vsl, SLT_ExpKill, "LRU_Fail");
@@ -3227,6 +3233,10 @@ fellow_cache_lru_work(struct worker *wrk, struct fellow_cache_lru *lru)
 		return (0);
 	}
 
+	/*
+	 * at this point, we know we terminated the foreach
+	 * with a break, so we either have an alloc return, or an FCO
+	 */
 	if (alloc.ptr != NULL) {
 		AZ(fco);
 		AN(alloc.size);