Documentation change: Showed the difference between map and and_then with an example.

[sandeep-datta]

Updated documentation to clarify the difference between and_then and map. This also explains why we need and_then in addition to map. Please look at the diff for more information.

[rust-highfive]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @brson (or someone else) soon.

If any changes to this PR are deemed necessary, please add them as extra commits. This ensures that the reviewer can see what has changed since they last reviewed the code. Due to the way GitHub handles out-of-date commits, this should also make it reasonably obvious what issues have or haven't been addressed. Large or tricky changes may require several passes of review and changes.

Please see the contribution instructions for more information.

[nagisa]

r? @steveklabnik

[alexcrichton]

This looks like it may have picked up some extra commits by accident as well, could you try a git rebase?

[sandeep-datta]

Sure thing! Give me some time.

[sandeep-datta]

@alexcrichton I am sorry I had to create a new pull request to get rid of the unwanted commits. I have closed this pull request. Please look at #30971 now.