docs(LifecycleHooks): correct ngDoCheck description

[zbjornson]

Please check if the PR fulfills these requirements

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

What kind of change does this PR introduce? (Bug fix, feature, docs update, ...)

• Bugfix
• Feature
• Code style update (formatting, local variables)
• Refactoring (no functional changes, no api changes)
• Build related changes
• CI related changes
• Other... Please describe: Doc fix

What is the current behavior? (You can also link to an open issue here)
Docs describe incorrect behavior. (See #6810 and #7307.)

What is the new behavior?
Docs describe current actual behavior.

Does this PR introduce a breaking change?

• Yes
• No

Other information:

The current behavior is for ngDoCheck to supplement, not override, the default change detector. OnChanges will still be called when DoCheck is implemented (fixes #7307, #6810).

I assume the desired behavior of ngDoCheck has been discussed. What the docs described actually sounds nice, both from a usage and performance perspective, but @mhevery's comment in #7307 suggests that the current behavior is as-intended.

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