dj.1: -A and -a: fix descriptions

docs/dj.1

@@ -45,11 +45,13 @@ dj
 .B output offset
 .R ])
 
-.SH USAGE
+.SH OPTIONS
 
 .B -A
 .RS
-Takes no arguments and pads with nuls.
+If the output is a stream, nul bytes are printed. In other words, it does what
+.B -a
+does but with null bytes instead.
 .RE
 
 .B -B
@@ -74,8 +76,8 @@ output is a stream, nul characters are printed.
 
 .B -a
 .RS
-Takes one argument of one byte in length and pads the input buffer with
-that byte in the event that a read doesn’t fill the input buffer, and the
+Takes one argument of one byte in length and pads the input buffer with it in
+the event of an incomplete read from the input file.
 .RE
 
 .B -b