message: Update docs about TermDict.
diff: use common::{BinarySerializable, FixedSize};
  use std::io;
  -/// `TermInfo` contains all of the information
  -/// associated to terms in the `.term` file.
  -///
  -/// It consists of
  -/// * `doc_freq` : the number of document in the segment
  -/// containing this term. It is also the length of the
  -/// posting list associated to this term
  -/// * `postings_offset` : an offset in the `.idx` file
  -/// addressing the start of the posting list associated
  -/// to this term.
  +/// `TermInfo` wraps the metadata associated to a Term.
  +/// It is segment-local.
  #[derive(Debug, Default, Ord, PartialOrd, Eq, PartialEq, Clone)]
  pub struct TermInfo {
  /// Number of documents in the segment containing the term
  pub doc_freq: u32,
  - /// Offset within the postings (`.idx`) file.
  + /// Start offset within the postings (`.idx`) file.
  pub postings_offset: u64,
  - /// Offset within the position (`.pos`) file.
  + /// Start offset of the first block within the position (`.pos`) file.
  pub positions_offset: u64,
  - /// Offset within the position block.
  + /// Start offset within this position block.
  pub positions_inner_offset: u8,
  }
  impl FixedSize for TermInfo {
  - /// Size required for the binary serialization of `TermInfo`.
  - /// This is large, but in practise, all `TermInfo` but the first one
  - /// of the block are bitpacked.
  - ///
  - /// See `TermInfoStore`.
  + /// Size required for the binary serialization of a `TermInfo` object.
  + /// This is large, but in practise, `TermInfo` are encoded in blocks and
  + /// only the first `TermInfo` of a block is serialized uncompressed.
  + /// The subsequent `TermInfo` are delta encoded and bitpacked.
  const SIZE_IN_BYTES: usize = u32::SIZE_IN_BYTES + 2 * u64::SIZE_IN_BYTES + u8::SIZE_IN_BYTES;
  }
  