docs(package_info_plus): Improve web documentation re. CORS

[miquelbeltran]

Description

Adds documentation regarding how the plugin works in web and possible CORS access issues.

Related Issues

• Related [Bug]: Null Package Version Web #2699 (comment)

Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I titled the PR using Conventional Commits.
• I did not modify the CHANGELOG.md nor the plugin version in pubspec.yaml files.
• All existing and new tests are passing.
• The analyzer (flutter analyze) does not report any problems on my PR.

Breaking Change

Does your PR require plugin users to manually update their apps to accommodate your change?

• Yes, this is a breaking change (please indicate that with a ! in the title as explained in Conventional Commits).
• No, this is not a breaking change.