Update platform_specific_instructions.md: Add 'Why' explanations for better understanding

[AffanShaikhsurab]

Add explanations for platform-specific code to improve clarity for newcomers by detailing why the changes are necessary

PR Description

This PR enhances the platform_specific_instructions.md file by adding detailed explanations for platform-specific code changes. These explanations provide context on why certain modifications are required for macOS, Android, and Web platforms, helping newcomers understand the reasoning behind the code.

Changes Made:

1. macOS: Added details on why entitlement keys are necessary to avoid network connection errors during app execution.
2. Android: Explained the purpose of enabling multiDexEnabled in build.gradle to support apps with a large number of methods.
3. Web: Provided an explanation for fixing the build error caused by the printing package, detailing the removal of .toJS for compatibility with the latest Dart runtime.

By adding these explanations, this PR ensures that new developers can better understand the purpose of platform-specific configurations and avoid common pitfalls during development.

• Closes #

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?

We encourage you to add relevant test cases.

• Yes
• [ ✔️] No, and this is why: This PR involves documentation updates only, which do not require test cases.

OS on which you have developed and tested the feature?

• [ ✔️] Windows
• [] macOS
• [✔️ ] Linux

[ashitaprasad]

Thanks for the PR.
Merged under issue #527