Add missing doc comments.

src/mem/alloc.h

@@ -70,7 +70,15 @@ namespace snmalloc
    */
   struct SuperslabMap
   {
+    /**
+     * Reference to the pagemap that we're using, allowing it to not be the
+     * default pagemap.
+     */
     SuperslabPagemap& pagemap;
+    /**
+     * Constructor.  Uses the global pagemap by default, but can use a
+     * different one if required.
+     */
     SuperslabMap(SuperslabPagemap& pm = global_pagemap) : pagemap(pm) {}
     /**
      * Get the pagemap entry corresponding to a specific address.

src/mem/pagemap.h

@@ -198,8 +198,20 @@ namespace snmalloc
     }
 
   public:
+    /**
+     * The pagemap configuration describing this instantiation of the template.
+     */
     static constexpr PagemapConfig config = {
       1, false, GRANULARITY_BITS, sizeof(T)};
+
+    /**
+     * Cast a `void*` to a pointer to this template instantiation, given a
+     * config describing the configuration.  Return null if the configuration
+     * passed does not correspond to this template instantiation.
+     *
+     * This intended to allow code that depends on the pagemap having a
+     * specific representation to fail gracefully.
+     */
     static Pagemap* cast_to_pagemap(void* pm, const PagemapConfig* c)
     {
       if (
@@ -290,9 +302,20 @@ namespace snmalloc
     std::atomic<T> top[ENTRIES];
 
   public:
+    /**
+     * The pagemap configuration describing this instantiation of the template.
+     */
     static constexpr PagemapConfig config = {
       1, true, GRANULARITY_BITS, sizeof(T)};
 
+    /**
+     * Cast a `void*` to a pointer to this template instantiation, given a
+     * config describing the configuration.  Return null if the configuration
+     * passed does not correspond to this template instantiation.
+     *
+     * This intended to allow code that depends on the pagemap having a
+     * specific representation to fail gracefully.
+     */
     static FlatPagemap* cast_to_pagemap(void* pm, PagemapConfig* c)
     {
       if (

src/override/malloc.cc

@@ -206,6 +206,14 @@ extern "C"
   }
 
 #ifdef SNMALLOC_EXPOSE_PAGEMAP
+  /**
+   * Export the pagemap.  The return value is a pointer to the pagemap
+   * structure.  The argument is used to return a pointer to a `PagemapConfig`
+   * structure describing the type of the pagemap.  Static methods on the
+   * concrete pagemap templates can then be used to safely cast the return from
+   * this function to the correct type.  This allows us to preserve some
+   * semblance of ABI safety via a pure C API.
+   */
   SNMALLOC_EXPORT void* SNMALLOC_NAME_MANGLE(snmalloc_get_global_pagemap)(
     PagemapConfig const** config)
   {