Minor: rename `GetFieldAccessCharacteristic` and add docs #2087

Job	Run time
cargo test (mac)	5m 26s
Verify MSRV	10m 12s
cargo check	4m 8s
Check cargo fmt	45s
Verify Vendored Code	1m 0s
cargo test (win64)	4m 37s
cargo test doc (amd64)	0s
cargo doc	0s
cargo test (amd64)	0s
cargo test datafusion-cli (amd64)	0s
cargo examples (amd64)	0s
Run sqllogictest with Postgres runner	0s
verify benchmark results (amd64)	0s
check Cargo.toml formatting	0s
clippy	0s
check configs.md is up-to-date	0s
cargo test hash collisions (amd64)	0s
cargo test pyarrow (amd64)	0s
	26m 8s

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