Method ID documentation in FunC and TVM

[anton-trunov]

Is your feature request related to a problem? Please describe.

There is an explanation how method IDs are calculated if not set explicitly https://docs.ton.org/develop/func/functions#method_id, but what are the restrictions on method ID values if the programmer sets it by hand?

For instance, FunC allows method IDs to be larger than 257-bit integers:

int %foo() method_id(0x6ffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffff) {
    return 42;
}

But on the other hand there is a limit, so the following

int %foo() method_id(0x7ffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffff) {
    return 42;
}

throws an error like so:

method-id.fc:1:22: error: invalid integer constant `0x7ffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffff`

Describe the solution you'd like

1. Document the exact range for manually specified method IDs in Func
2. Explain the allowed range for method IDs (DECLMETHOD indices) in TVM

Additional context

No response

[anton-trunov]

@NickNekilov mentions only 19 bits are used:

https://github.com/ton-blockchain/ton/blob/921aa29eb54db42de21e0f89610c347670988ed1/crypto/fift/lib/Asm.fif#L1483