Cleanup: move doc-strings to declarations

Move into headers or to the top of the function body for internal
implementation details, in some cases remove duplicate doc-strings.

intern/guardedalloc/intern/mallocn_intern.hh

@@ -93,6 +93,15 @@ void memory_usage_block_alloc(size_t size);
 void memory_usage_block_free(size_t size);
 size_t memory_usage_block_num(void);
 size_t memory_usage_current(void);
+/**
+ * Get the approximate peak memory usage since the last call to #memory_usage_peak_reset.
+ * This is approximate, because the peak usage is not updated after every allocation (see
+ * #peak_update_threshold).
+ *
+ * In the worst case, the peak memory usage is underestimated by
+ * `peak_update_threshold * #threads`. After large allocations (larger than the threshold), the
+ * peak usage is always updated so those allocations will always be taken into account.
+ */
 size_t memory_usage_peak(void);
 void memory_usage_peak_reset(void);
 

intern/guardedalloc/intern/mallocn_lockfree_impl.cc

@@ -515,7 +515,8 @@ void MEM_lockfree_printmemlist() {}
 
 void mem_lockfree_clearmemlist() {}
 
-/* unused */
+/* Unused. */
+
 void MEM_lockfree_callbackmemlist(void (*func)(void *))
 {
   (void)func; /* Ignored. */
@@ -560,7 +561,8 @@ uint MEM_lockfree_get_memory_blocks_in_use()
   return uint(memory_usage_block_num());
 }
 
-/* dummy */
+/* Dummy. */
+
 void MEM_lockfree_reset_peak_memory()
 {
   memory_usage_peak_reset();

intern/guardedalloc/intern/memory_usage.cc

@@ -252,15 +252,6 @@ size_t memory_usage_current()
   return size_t(mem_in_use);
 }
 
-/**
- * Get the approximate peak memory usage since the last call to #memory_usage_peak_reset.
- * This is approximate, because the peak usage is not updated after every allocation (see
- * #peak_update_threshold).
- *
- * In the worst case, the peak memory usage is underestimated by
- * `peak_update_threshold * #threads`. After large allocations (larger than the threshold), the
- * peak usage is always updated so those allocations will always be taken into account.
- */
 size_t memory_usage_peak()
 {
   update_global_peak();