KAFKA-19877: Clarify ByteBuffer position and limit expectations for Deserializer

[Shally-Katariya]

Motivation

The Deserializer interface currently does not clearly specify how a ByteBuffer's
position and limit should be treated during and after deserialization.
This ambiguity can lead to inconsistent implementations and incorrect assumptions
by callers.

Changes

• Clarified the contract for Deserializer#deserialize(String, Headers, ByteBuffer)
  to state that implementations should read from the buffer’s current position,
  may advance the position by the number of bytes consumed, and must not modify
  the buffer limit.
• Documented that callers should not make assumptions about the buffer position
  after deserialization.

Notes

This change is documentation-only and does not modify runtime behavior.