[dajiaohuang]

Closes #1349.

Summary

• Explain that expr() builds the repository's Expr representation and is not a parser for the full first-order grammar.
• Clarify the shared call representation for relation-like atoms and function-like terms, Python structural equality versus logical equality, and the definite-clause/implicit-universal scope of FolKB.
• Add synthetic examples showing expression trees and a query substitution.

Validation

• Parsed the notebook JSON and executed the two added synthetic cells against the checked-out aima.logic; outputs matched the notebook.
• git diff --check passed.
• Did not run the full notebook or test suite; CI was not queried.

Only notebooks/logic.ipynb was changed. The notebook's generated .py mirror was left untouched. The examples are original and do not reproduce textbook passages.