Migrate documentation for `regr*` aggregate functions to code #33863

Job	Run time
Check cargo fmt	43s
cargo check	6m 54s
build with wasm-pack	2m 35s
cargo test (macos)	11m 37s
cargo test (macos-aarch64)	9m 56s
Verify MSRV (Min Supported Rust Version)	9m 20s
Verify Vendored Code	50s
cargo test (win64)	1h 1m 32s
cargo test (amd64)	10m 57s
cargo doc	3m 59s
cargo examples (amd64)	13m 40s
cargo test datafusion-cli (amd64)	4m 1s
cargo test doc (amd64)	15m 11s
verify benchmark results (amd64)	12m 51s
clippy	5m 43s
check Cargo.toml formatting	3m 8s
check configs.md and ***_functions.md is up-to-date	3m 3s
cargo test hash collisions (amd64)	17m 8s
Run sqllogictest with Postgres runner	3m 34s
cargo test pyarrow (amd64)	2m 46s
	3h 19m 28s

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