[docs] Add Google-style docstrings for Signature field-editing methods

[gnetsanet]

What feature would you like to see?

Following issue #8926, this ticket tracks docstring additions for:

• Signature.prepend
• Signature.append
• Signature.insert
• Signature.delete

I plan to add concise, factual Google-style docstrings and verify behavior.

Would you like to contribute?

• Yes, I'd like to help implement this.
• No, I just want to request it.

Additional Context

No response

[gnetsanet]

Opened PR #8945 for Signature field-editing methods.