Add (more) Parquet Metadata Documentation (#6184)

* Minor: Add (more) Parquet Metadata Documenation

* fix clippy

parquet/src/file/metadata/mod.rs

@@ -33,6 +33,67 @@
 //! * [`ColumnChunkMetaData`]: Metadata for each column chunk (primitive leaf)
 //!   within a Row Group including encoding and compression information,
 //!   number of values, statistics, etc.
+//!
+//! # APIs for working with Parquet Metadata
+//!
+//! The Parquet readers and writers in this crate read and write
+//! metadata into parquet files. To work with metadata directly,
+//! the following APIs are available.
+//!
+//! Reading:
+//! * Read from bytes to `ParquetMetaData`: [`decode_footer`]
+//!   and [`decode_metadata`]
+//! * Read from an `async` source to `ParquetMetadata`: [`MetadataLoader`]
+//!
+//! [`MetadataLoader`]: https://docs.rs/parquet/latest/parquet/arrow/async_reader/struct.MetadataLoader.html
+//! [`decode_footer`]: crate::file::footer::decode_footer
+//! [`decode_metadata`]: crate::file::footer::decode_metadata
+//!
+//! Writing:
+//! * Write `ParquetMetaData` to bytes in memory: Not yet supported (see [#6002])
+//! * Writes `ParquetMetaData` to an async target: Not yet supported
+//!
+//! [#6002]: https://github.com/apache/arrow-rs/issues/6002
+//!
+//! # Metadata Encodings and Structures
+//!
+//! There are three different encodings of Parquet Metadata in this crate:
+//!
+//! 1. `bytes`:encoded with the Thrift TCompactProtocol as defined in
+//!    [parquet.thrift]
+//!
+//! 2. [`format`]: Rust structures automatically generated by the thrift compiler
+//!    from [parquet.thrift]. These structures are low level and mirror
+//!    the thrift definitions.
+//!
+//! 3. [`file::metadata`] (this module): Easier to use Rust structures
+//!    with a more idiomatic API. Note that, confusingly, some but not all
+//!    of these structures have the same name as the [`format`] structures.
+//!
+//! [`format`]: crate::format
+//! [`file::metadata`]: crate::file::metadata
+//! [parquet.thrift]:  https://github.com/apache/parquet-format/blob/master/src/main/thrift/parquet.thrift
+//!
+//! Graphically, this is how the different structures relate to each other:
+//!
+//! ```text
+//!                          ┌ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─         ┌ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─
+//!                            ┌──────────────┐     │         ┌───────────────────────┐ │
+//!                          │ │ ColumnIndex  │              ││    ParquetMetaData    │
+//!                            └──────────────┘     │         └───────────────────────┘ │
+//! ┌──────────────┐         │ ┌────────────────┐            │┌───────────────────────┐
+//! │   ..0x24..   │ ◀────▶    │  OffsetIndex   │   │ ◀────▶  │    ParquetMetaData    │ │
+//! └──────────────┘         │ └────────────────┘            │└───────────────────────┘
+//!                                     ...         │                   ...             │
+//!                          │ ┌──────────────────┐          │ ┌──────────────────┐
+//! bytes                      │  FileMetaData*   │ │          │  FileMetaData*   │     │
+//! (thrift encoded)         │ └──────────────────┘          │ └──────────────────┘
+//!                           ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ┘         ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ┘
+//!
+//!                          format::meta structures          file::metadata structures
+//!
+//!                         * Same name, different struct
+//! ```
 mod memory;
 
 use std::ops::Range;