docs: add more details on implicit differentiation

[Benjamin-eecs]

Description

add more docs on implicit differentiation

Types of changes

• Documentation (update in the documentation)

Checklist

• I have read the CONTRIBUTION guide. (required)
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have reformatted the code using make format. (required)
• I have checked the code using make lint. (required)
• I have ensured make test pass. (required)

[XuehaiPan]

Misunderstanding for $T$ and $F$.