Web instructions update

[Aman071106]

PR Description

This PR adds a clarification warning to the documentation regarding the temporary fix for removing .toJS() from printing_web.dart in the printing package. Recent Dart versions no longer require .toJS() for converting Dart strings to JavaScript strings, as reflected in the upstream change proposed in PR #1739.

Because the temporary workaround involves modifying files inside the user's local .pub-cache, this update adds a clear warning block explaining that such changes are not persistent and will be overwritten when:

• running flutter pub get
• clearing the pub cache
• upgrading packages
• setting up the project on a different machine

This improves documentation clarity and prevents confusion for users applying local patches.

Related Issues

• (Doc update) Documentation contributions are welcome! #527

Checklist

• I have gone through the contributing guide
• I have updated my branch and synced it with project main branch before making this PR
• I am using the latest Flutter stable branch (run flutter upgrade and verify)
• I have run the tests (flutter test) and all tests are passing

Added/updated tests?

• Yes
• No - documentation-only update, tests not required.

OS on which you have developed and tested the feature?

• Windows
• macOS
• Linux

[animator]

Thanks.