Updated documented JSON API encoding for decimals

[HilaryYates]

Fixes #15302

I updated the JSON API docs for decimals, which were outdated. Now, it states (as per LF numerics ), that the fixpoint can be anywhere within the digits except at the start.

see https://github.com/digital-asset/daml/blob/main/daml-lf/transaction/src/main/protobuf/com/daml/lf/value.proto#L39-L48

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[remyhaemmerle-da]

Thanks for taking care of thank, however :

• As far I know this is not use by the JSON API. It is use to persists value in the DB and communicate data between participant.
• Since LF: Kill deprecated Transaction versions #7785 (basically SDK 2.0) we do not support anymore versions 1 to 5, so the comment is now useless. It is probably better to just drop it complelty.

[remyhaemmerle-da]

Please:

• Drop the comment complelty
• Change the title to something like "[LF] Drop outdated documentation in Ledger values proto."