[Ada] Minor comment improvement

2018-05-31  Ed Schonberg  <schonberg@adacore.com>

gcc/ada/

	* einfo.ads: Improve documentation for the Is_Unsigned_Type entity
	flag.

From-SVN: r261004

gcc/ada/ChangeLog

+2018-05-31  Ed Schonberg  <schonberg@adacore.com>
+
+	* einfo.ads: Improve documentation for the Is_Unsigned_Type entity
+	flag.
+
 2018-05-31  Piotr Trojanek  <trojanek@adacore.com>
 
 	* sem_prag.adb (Analyze_Pragma): Replace call to Unique_Defining_Entity

gcc/ada/einfo.ads

@@ -3338,6 +3338,10 @@ package Einfo is
 --       a further subtype is obtained using variable bounds. This further
 --       subtype is still unsigned, but this cannot be determined by looking
 --       at its bounds or the bounds of the corresponding base type.
+--       For a subtype indication whose range is statically a null range,
+--       the flag is set if the lower bound is non-negative, but the flag
+--       cannot be used to determine the comparison operator to emit in the
+--       generated code: use the base type.
 
 --    Is_Uplevel_Referenced_Entity (Flag283)
 --       Defined in all entities. Used when unnesting subprograms to indicate