docs(express.json): clarify usage and add route-specific limit example

[kushal4345]

Summary

This PR updates the express.json() documentation to:

• Clarify behavior and options in a more concise way.
• Add an example showing how to set different limit values for specific routes (e.g., /upload).

Changes

• Added a "Different limits for specific routes" section with a code example.
• Maintained existing style and formatting consistent with the Express docs.
• Verified that markdown tables and headings render correctly.

Reason

Issue #1421 requests additional clarity on using express.json() with different route limits.
This helps developers quickly understand how to override defaults for specific endpoints without affecting others.

Notes

• No changes to Express codebase, only to documentation.
• Follows current contributing and formatting guidelines.

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