Add missing text to Prompt vesioning section in AI > Best practices > Prompting page

[victoreronmosele]

Description of what this PR is changing or adding, and why:

The Prompt versioning section in the AI > Best practices > Prompting page has incomplete text.

This PR adds the missing text from the Flutter AI best practices Google doc.

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After

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• This PR follows the Google Developer Documentation Style Guidelines—for example, it doesn't use i.e. or e.g., and it avoids I and we (first-person pronouns).
• This PR uses semantic line breaks
  of 80 characters or fewer.