bcachefs: document btree iter flags

[dlrobertson]

Add documentation commets to the btree iter flag definitions.

CC: https://github.com/koverstreet/bcachefs/issues/269

Signed-off-by: Dan Robertson [email protected]

[dlrobertson]

The style of writing you're using here would be better for a separate document

I can move this to the wiki and close this if that is preferred.

[koverstreet]

We can do both - we should be adding comments here, I'd just prefer to leave it to the more non obvious, higher value comments.