docs: (imgtool) explain --pad-header + hex file
Add an explanation for what happens when using the "--pad-header" flag
with a hex file.
@@ -79,7 +79,11 @@ flash device. For Zephyr, it will be configured as part of the build,
and will be a small power of two. By default, the Zephyr build system will
already prepended a zeroed header to the image. If another build system is
in use that does not automatically add this zeroed header, `--pad-header` can
-be passed and the `--header-size` will be added by imgtool.
+be passed and the `--header-size` will be added by imgtool. If `--pad-header`
+is used with an Intel Hex file, `--header-size` bytes will be subtracted from
+the load address (in Intel Hex terms, the Extended Linear Address record) to
+adjust for the new bytes prepended to the file. The load address of all data
+existing in the file should not change.
The `--slot-size` argument is required and used to check that the firmware
does not overflow into the swap status area (metadata). If swap upgrades are
