docs(auth): specify big-endian byte order for timestamp in HMAC input

[VasilevNStas]

Description

Specifies the byte order for timestamp serialization in the HMAC input for Auth.hmac. No wire changes.

Problem

The hmac comment states the tag is computed over (client_id || timestamp), where timestamp is a uint32. Proto3 varint encoding applies only to the wire format; the HMAC input is a raw byte string assembled outside any wire format. Without an explicit byte order, Go (big-endian convention) and Swift/Kotlin (little-endian) implementations produce different HMAC inputs for the same (client_id, timestamp) pair, causing silent authentication failures.

Fix

Added one sentence: "The timestamp is serialised as 4 bytes in big-endian (network byte order) before concatenation."

Closes #11

[VasilevNStas]

@davvd plz review

[davvd]

@VasilevNStas we have merge conflicts here, please resolve them and we'll merge

[VasilevNStas]

@davvd conflicts resolved.

[davvd]

@VasilevNStas conflict here

[VasilevNStas]

@davvd
I closed this PR because PR #58 (#58) already covers the same HMAC big-endian documentation, but with the correct byte count — since PR #58 changes `timestamp` from `uint32` (4 bytes) to `uint64` (8 bytes), the serialization is now 8 bytes in big-endian. Keeping PR #43 open would have required updating 4→8, which is already done in PR #58

The change in PR #58 is the same as what PR #43 intended: documenting that the timestamp in HMAC input is serialised as fixed-width big-endian. PR #58 just has the updated byte count for the new uint64 wire format