arrow

ARROW-17512: [Doc] Updates to crossbow documentation for clarity (#13993)

## Overview
While setting up a queue repository for testing changes to `crossbow`, we noticed some updates that can be made to help future developers set up their environment. 

## Implementation
1. Clarify Travis CI auto-cancellation default behavior.
2. Fix broken links referenced by instructions.
3. Minor typos.

## Testing
1. Qualified by performing a directory level sphinx build and visually verifying the changes.

## Notes
Thank you for your help on this pull request, @kevingurney!

Lead-authored-by: Fiona La <fionala7@gmail.com>
Co-authored-by: Kevin Gurney <kgurney@mathworks.com>
Signed-off-by: Antoine Pitrou <antoine@python.org>