[DomCrawler] Added missing docblocks and removed unneeded return annotation.
merged branch jakzal/domcrawler-missing-docblocks (PR #9025)

This PR was merged into the 2.2 branch.

Discussion
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[DomCrawler] Added missing docblocks and removed redundant type in a return annotation

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | -
| License       | MIT
| Doc PR        | -

Commits
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d414213 [DomCrawler] Added missing docblocks and removed unneeded return annotation.