[sridharkalaibala]

Summary

Documents how to use a function as replace in legacy lang/output extensions: callback arguments, return value, global matching, and the distinction from filter. Adds a complete example that transforms two captured words while preserving the existing recommendation to use listener extensions for new code.

Fixes #1033.

Validation

• Executed the documented example against the source-built Showdown bundle; verified both replacements and unchanged nonmatching text.
• Checked callback arguments, optional/named captures, match offset, and literal $1 return behavior against that bundle.
• npm test: 3,174 tests passed; lint has zero errors and one existing unused-variable warning in decodeEntities.js.
• Strict MkDocs build passed; verified the generated section, example output, and filter link.
• git diff --check passed.