[Documentation] Update SeedQR docs to note we assume the English bip39 wordlist is being used

[kdmukai]

SeedQR could trivially be used with any of the bip39 wordlists in other languages.

However there is no way to know which language was intended when interpreting a SeedQR. Interpreting a SeedQR while assuming the wrong intended language wordlist will result in a different or invalid seed.

The SeedQR docs should clarify that we are assuming -- and only recommend -- using the English wordlist.

[jdlcdl]

Mental note:

I think of the above as "BIP39en" and wish that all future wallets would adopt it, seems like they're doing so. It's the same as BIP39 with the restriction that when encoding entropy to mnemonic for hashing/stretching into 64byte seed, the wordlist is ALWAYS the well entrenched english list.

It would make for a better base-assumption, so that extensions could later be defined per locale, say "BIP39en+sw" for swahili (where swahili is decoded to entropy then encoded to english for hashing into seed). Maybe down the road becoming optional locale-plugins -- upgrade-packs w/ fonts, l10n catalogs, wordlists, keyboards. This way, any user can have optimal ui and meaningful backups, without the bulk of unwanted non-english locales on tiny devices.