docs(service-worker): mention that dataGroups only cache non-mutating requests

[jbogarthyde]

PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines: https://github.com/angular/angular/blob/master/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?

Service Worker doc does not mention that only non-mutating requests (GET and HEAD) are cached.

Issue Number: #28988

What is the new behavior?

Limitation is mentioned in https://angular.io/guide/service-worker-config#datagroups

Does this PR introduce a breaking change?

• Yes
• No

[mary-poppins]

You can preview fd84898 at https://pr32142-fd84898.ngbuilds.io/.

[gkalpak]

I would change the commit message to something like:

docs(service-worker): mention that dataGroups only cache non-mutating requests

[mary-poppins]

You can preview 1cfec06 at https://pr32142-1cfec06.ngbuilds.io/.

[mary-poppins]

You can preview 1fdfac0 at https://pr32142-1fdfac0.ngbuilds.io/.

[gkalpak]

Sorry, I forgot to mention it before:
You need to mention that this fixes #28988 in the commit message:

docs(service-worker): mention that dataGroups only cache non-mutating requests

Fixes #28988

[mary-poppins]

You can preview bef8b5e at https://pr32142-bef8b5e.ngbuilds.io/.

[mary-poppins]

You can preview 78381a5 at https://pr32142-78381a5.ngbuilds.io/.

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