Migrate documentation for regr* functions to code #3944

Job	Run time
cargo check	10m 43s
build with wasm-pack	2m 41s
Check cargo fmt	43s
cargo test (macos)	9m 46s
cargo test (macos-aarch64)	10m 34s
Verify MSRV (Min Supported Rust Version)	8m 20s
Verify Vendored Code	52s
cargo test (win64)	1h 2m 51s
cargo test (amd64)	11m 26s
cargo doc	4m 4s
clippy	5m 40s
cargo test doc (amd64)	15m 33s
cargo test datafusion-cli (amd64)	3m 55s
cargo examples (amd64)	13m 46s
Run sqllogictest with Postgres runner	3m 41s
verify benchmark results (amd64)	12m 41s
cargo test pyarrow (amd64)	2m 56s
check Cargo.toml formatting	3m 5s
check configs.md and ***_functions.md is up-to-date	2m 59s
cargo test hash collisions (amd64)	17m 29s
	3h 23m 45s

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