Document modify configuration syntax

src/doc/avrdude.texi

@@ -3822,6 +3822,25 @@ programmer
 If a parent is specified, all settings of it (except its ids) are used for the new
 programmer. These values can be changed by new setting them for the new programmer.
 
+@noindent
+Programmer definitions can be modified using the following syntax
+@smallexample
+modify programmer <id>
+    # Any property as above, for example
+    baudrate = <num>;                        # baudrate for the programmer
+    usbsn    = <serialno>;                   # USB Serial Number
+;
+@end smallexample
+
+@noindent
+The @code{modify} keyword makes most sense in the per-user configuration
+file, where users may want to add specific properties of their programmer
+such as its USB serial number. This technique should not be used for the
+main @code{avrdude.conf} file, as the developer option @code{-c
+<programmer>/S} that prints the programmer definition as AVRDUDE
+understands it will only consider the final definition; it will not print
+@code{modify} statements.
+
 @noindent
 @strong{Notes}
 
@@ -4092,6 +4111,23 @@ exception of pin numbers and where they are separated by space, e.g., in
 signature and readback) can also be given as simple expressions involving
 arithemtic and bitwise operators.
 
+@noindent
+Part definitions can be modified using the following syntax
+@smallexample
+modify part <id>
+    # Any part property as above, for example
+    chip_erase_delay = <num>;                 # microseconds
+;
+@end smallexample
+
+@noindent
+The @code{modify} keyword makes most sense in the per-user configuration
+file, where users may want to modify specific properties of a part for
+debugging. This technique should not be used within the main
+@code{avrdude.conf} file, as the developer option @code{-p <part>/S} that
+prints the part definition as AVRDUDE understands it will only consider
+the final definition; it will not print @code{modify} statements.
+
 @menu
 * Parent Part::
 * Instruction Format::