Fix Haddock documentation for fromAscListWithKey functions

[RikuMinamiyama]

Overview
This PR fixes Haddock documentation issues in the fromAscListWithKey function documentation.

Correction details

• Fix missing backticks in 'fromAscListWithKey' reference in fromListWithKey documentation
• Improve English phrasing: "on equal keys" → "for equal keys"
• Clarify that the combining function receives the key: "with a combining function that receives the key for equal keys"
• Enhance examples with more comprehensive test cases including empty list
• Apply consistent changes to both Strict and Lazy versions

Modified files

• containers/src/Data/IntMap/Internal.hs
• containers/src/Data/IntMap/Strict/Internal.hs

Test

• Documentation builds without errors
• Changes are consistent between Strict and Lazy versions
• Examples are syntactically correct

This is my first OSS contribution and I'm not a native English speaker. Please let me know if there are any issues with the documentation changes.

Fixes #1154