Add note about "legacy" format to be used for OTA update from access point WebGUI

Hello!

When visiting a device via its access point, there can be an option to upload a file for an OTA update.

However, if the user accidentally selects the file in the "modern" (Factory) and not the "legacy" (OTA) format, the OTA update will fail with:

`Update Failed: Wrong Magic Byte`

It took me a while to find [this](https://github.com/esphome/esphome/discussions/6317#discussioncomment-8662714):
> Serial flash uses the "modern" format. OTA update uses the "legacy" format.

See related discussion:
https://github.com/esphome/esphome/discussions/6317

Is it possible to add a note in this regard on the WebGUI that comes along with the AP and "OTA Update" section? And/or provide the detail with the aforementioned error message, e.g.:

`Update Failed: Wrong Magic Byte - check correct OTA vs. Factory format`

Thanks for your kind consideration and for upvoting it if you feel it is worth it👍