doc: add documentation on naming functions in a Kptfile

[michaelvl]

This PR documents/clarifies that using name on functions in a Kptfile is important when updating packages with kpt pkg update. The issue was discussed in Slack here: https://kubernetes.slack.com/archives/C0155NSPJSZ/p1689926397556699

The name field was not mentioned in the current version of the book, and since function lists are merged as associative lists, the effect of not specifying a name may result in unpredictable ordering of the function lists.

Specifically, the PR updates section 4.1 of the kpt book on "Declarative Function Execution"

A few considerations for reviewers:

• This document https://github.com/GoogleContainerTools/kpt/pull/2623/files deliberately avoids function names with spaces and resorts to names-with-hyphens instead. This PR use an example with spaces in names.
• The same document also describe cases where it is possible to do function identification based on the value of image. This PR does not describe this situation since there are multiple requirements for this to be possible. Also, I'm suspecting that this functionality may no longer be active since the example shown in the Slack thread should have merged the updated function list if it had used image as identifier.

[google-cla]

Thanks for your pull request! It looks like this may be your first contribution to a Google open source project. Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

View this failed invocation of the CLA check for more information.

For the most up to date status, view the checks section at the bottom of the pull request.

[natasha41575]

Looks pretty good. Some small wording suggestions.

[natasha41575]

Thanks! Will merge this once the tests finish.