Updated the Using nextTick section

[bhaskar-nair2]

1. The currrent description for Using nextTick was very confusing for a beginner so updated that for easier understanding.
2. The example provided for nextTick would have not worked unless the user would import Vue (which had not been stated anywhere so far, nor was necessary) so made changes for the example to work with wrapper.vm.$nextTick().

What kind of change does this PR introduce? (check at least one)

• Bugfix
• Feature
• Code style update
• Refactor
• Build-related changes
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Does this PR introduce a breaking change? (check one)

• Yes
• No

If yes, please describe the impact and migration path for existing applications:

The PR fulfills these requirements:

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• New/updated tests are included

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• A convincing reason for adding this feature (to avoid wasting your time, it's best to open a suggestion issue first and wait for approval before working on it)

Other information:

[JessicaSachs]

Nice. Thank you for this.

Context: @bhaskar-nair2 and I spoke in Vue Land's testing channel after he read the docs and needed additional context on what/why nextTick was needed.

This helps users who are visiting the docs/VTU for the first time understand WHY and WHEN they will need nextTick.