Document the semantic of ranges.

* libmisc/idmapping.h: Document what the upper and lower fields
	are in struct map_range.
	* man/newgidmap.1.xml: Document when the gid, gidlower and count
	argument are.
	* man/newuidmap.1.xml: Likewise for uid, uidlower and count.

libmisc/idmapping.h

@@ -31,9 +31,9 @@
 #define _IDMAPPING_H_
 
 struct map_range {
-	unsigned long upper;
-	unsigned long lower;
-	unsigned long count;
+	unsigned long upper; /* first ID inside the namespace */
+	unsigned long lower; /* first ID outside the namespace */
+	unsigned long count; /* Length of the inside and outside ranges */
 };
 
 extern struct map_range *get_map_ranges(int ranges, int argc, char **argv);