[Ada] Update the character type comment in exp_dbug.ads

[Pierre-Marie de Rodat <derodat@adacore.com>]

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The character type comment in exp_dbug.ads documented the QU and QW
encodings, but did not document the plain "Qc" encoding, where 'c' is
the character itself.  This patch updates the comment to follow the
implementation.

Tested on x86_64-pc-linux-gnu, committed on trunk

2019-09-17  Tom Tromey  <tromey@adacore.com>

gcc/ada/

	* exp_dbug.ads: Update character type comment.

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--- gcc/ada/exp_dbug.ads
+++ gcc/ada/exp_dbug.ads
@@ -1452,18 +1452,21 @@ package Exp_Dbug is
    --  a character literal, the name is encoded as described in the following
    --  paragraph.
 
-   --  A name QUhh, where each 'h' is a lower-case hexadecimal digit, stands
-   --  for a character whose Unicode encoding is hh, and QWhhhh likewise stands
-   --  for a wide character whose encoding is hhhh. The representation values
-   --  are encoded as for ordinary enumeration literals (and have no necessary
-   --  relationship to the values encoded in the names).
+   --  The characters 'a'..'z' and '0'..'9' are represented as Qc, where 'c'
+   --  stands for the character itself.  A name QUhh, where each 'h' is a
+   --  lower-case hexadecimal digit, stands for a character whose Unicode
+   --  encoding is hh, and QWhhhh likewise stands for a wide character whose
+   --  encoding is hhhh. The representation values are encoded as for ordinary
+   --  enumeration literals (and have no necessary relationship to the values
+   --  encoded in the names).
 
    --  For example, given the type declaration
 
-   --    type x is (A, 'C', B);
+   --    type x is (A, 'C', 'b');
 
    --  the second enumeration literal would be named QU43 and the value
-   --  assigned to it would be 1.
+   --  assigned to it would be 1, and the third enumeration literal would be
+   --  named Qb and the value assigned to it would be 2.
 
    -----------------------------------------------
    -- Secondary Dispatch tables of tagged types --