docs: explain difference between @placeholder and @loading

[eluchsinger]

Add a sentence to the @loading block documentation that emphasizes more that the @loading block will replace the @placeholder block once the deferred view starts loading.

PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines: https://github.com/angular/angular/blob/main/CONTRIBUTING.md#commit
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

• Bugfix
• Feature
• Code style update (formatting, local variables)
• Refactoring (no functional changes, no api changes)
• Build related changes
• CI related changes
• Documentation content changes
• angular.io application / infrastructure changes
• Other... Please describe:

What is the current behavior?

Issue Number: #54685

What is the new behavior?

Does this PR introduce a breaking change?

• Yes
• No

Other information

[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.

[JeanMeche]

Hi, thank you for your contribution, can you have a look at the CLA and sign it ? Thank you.

[eluchsinger]

@JeanMeche I have amended the commit with my Email that has the CLA signed.

[bencodezen]

Thanks for the PR @eluchsinger! I submitted a commit to improve the phrasing on the paragraph. Let me know what you think!

[JeanMeche]

@eluchsinger or @bencodezen can one of you squash the 2 commits ? And we're good to go !

[dylhunn]

This PR was merged into the repository by commit 9e60609.

[angular-automatic-lock-bot]

This issue has been automatically locked due to inactivity.
Please file a new issue if you are encountering a similar or related problem.

Read more about our automatic conversation locking policy.

_{This action has been performed automatically by a bot.}