docs(forms): add comment of the 'value' property

[lynette-li]

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?

Issue Number: 15982
#15982

According to the above issue,we can know that for an enabled FormGroup,the 'value' property only contain the enabled values of its children.Howerve,for a disabled FormGroup,the 'value' property contain all the values of its children.This should be written in the comment.

What is the new behavior?

Does this PR introduce a breaking change?

• Yes
• No

Other information

[googlebot]

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[brandonroberts]

@Ly-lns the CLA requirement must be resolved before this PR can move forward.

[googlebot]

CLAs look good, thanks!

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[lynette-li]

@brandonroberts thanks.The CLA is signed now!

[brandonroberts]

Caretaker note: This is a docs only change and should not impact g3. Please run presubmit.

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