frame-omni-bencher maintenance (#5466)

Changes:
- Set default level to `Info` again. Seems like a dependency update set
it to something higher.
- Fix docs to not use `--locked` since we rely on dependency bumps via
cargo.
- Add README with rust docs.
- Fix bug where the node ignored `--heap-pages` argument.

You can test the `--heap-pages` bug by running this command on master
and then on this branch. Note that it should fail because of the very
low heap pages arg:
`cargo run --release --bin polkadot --features=runtime-benchmarks --
benchmark pallet --chain=dev --steps=10 --repeat=30
--wasm-execution=compiled --heap-pages=8 --pallet=frame-system
--extrinsic="*"`

---------

Signed-off-by: Oliver Tale-Yazdi <oliver.tale-yazdi@parity.io>
Co-authored-by: ggwpez <ggwpez@users.noreply.github.com>