docs(forms): clarify note in reactive forms doc

[kapunahelewong]

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
[x] Documentation content changes
[ ] angular.io application / infrastructure changes
[ ] Other... Please describe:

What is the current behavior?

Issue Number: #20058

What is the new behavior?

Change formGroupName to [formGroup].

Does this PR introduce a breaking change?

[ ] Yes
[x] No

Other information

[kapunahelewong]

@kara Would you review when you get a chance? Thank you!

[kara]

LGTM

[IgorMinar]

please update the commit message. see #21110 (comment)

[IgorMinar]

@kara I shouldn't need to approve this particular PR. Can you please send a PR that adds forms tutorial and guide md files to the existing forms group? thanks!

[IgorMinar]

@kara I shouldn't need to approve this particular PR. Can you please send a PR that adds forms tutorial and guide md files to the existing forms group? thanks!

[alexeagle]

Haven't been able to get a green build here. Same reason as the other recent PR?

[kapunahelewong]

@alexeagle I can rebase, but I see the tests are running. I can hold on pushing until we see if it fails.

EDIT: It failed, so I've rebased and just pushed.

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