Document that 'h', 'o', and 'd' file formats are output-only.

src/avrdude.1

@@ -733,14 +733,17 @@ fuse bit settings.
 hexadecimal; each value will get the string
 .Em 0x
 prepended.
+Only valid on output.
 .It Ar o
 octal; each value will get a
 .Em 0
 prepended unless it is less than 8 in which case it gets no prefix.
+Only valid on output.
 .It Ar b
 binary; each value will get the string
 .Em 0b
 prepended.
+Only valid on output.
 .El
 .Pp
 The default is to use auto detection for input files, and raw binary

src/doc/avrdude.texi

@@ -802,13 +802,16 @@ fuse bit settings.
 
 @item h
 hexadecimal; each value will get the string @emph{0x} prepended.
+Only valid on output.
 
 @item o
 octal; each value will get a @emph{0}
 prepended unless it is less than 8 in which case it gets no prefix.
+Only valid on output.
 
 @item b
 binary; each value will get the string @emph{0b} prepended.
+Only valid on output.
 
 @end table