mm: accurately document nr_free_*_pages functions with code comments

nr_free_zone_pages(), nr_free_buffer_pages() and nr_free_pagecache_pages()
are horribly badly named, so accurately document them with code comments
in case of the misuse of them.

[akpm@linux-foundation.org: tweak comments]
Reviewed-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Zhang Yanfei <zhangyanfei@cn.fujitsu.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/page_alloc.c

@@ -2813,6 +2813,15 @@ void free_pages_exact(void *virt, size_t size)
 }
 EXPORT_SYMBOL(free_pages_exact);
 
+/**
+ * nr_free_zone_pages - count number of pages beyond high watermark
+ * @offset: The zone index of the highest zone
+ *
+ * nr_free_zone_pages() counts the number of counts pages which are beyond the
+ * high watermark within all zones at or below a given zone index.  For each
+ * zone, the number of pages is calculated as:
+ *     present_pages - high_pages
+ */
 static unsigned long nr_free_zone_pages(int offset)
 {
 	struct zoneref *z;
@@ -2833,8 +2842,11 @@ static unsigned long nr_free_zone_pages(int offset)
 	return sum;
 }
 
-/*
- * Amount of free RAM allocatable within ZONE_DMA and ZONE_NORMAL
+/**
+ * nr_free_buffer_pages - count number of pages beyond high watermark
+ *
+ * nr_free_buffer_pages() counts the number of pages which are beyond the high
+ * watermark within ZONE_DMA and ZONE_NORMAL.
  */
 unsigned long nr_free_buffer_pages(void)
 {
@@ -2842,8 +2854,11 @@ unsigned long nr_free_buffer_pages(void)
 }
 EXPORT_SYMBOL_GPL(nr_free_buffer_pages);
 
-/*
- * Amount of free RAM allocatable within all zones
+/**
+ * nr_free_pagecache_pages - count number of pages beyond high watermark
+ *
+ * nr_free_pagecache_pages() counts the number of pages which are beyond the
+ * high watermark within all zones.
  */
 unsigned long nr_free_pagecache_pages(void)
 {