doc: Fix doxygen comment for SignTransaction in rpc/rawtransaction_util

Summary:
The param coins to SignTransaction is final and can thus not be extended (as suggested by the doc).
This was changed in PR16798 where ParsePrevouts was extracted from SignTransaction.

This is a backport of Core PR16900

Test Plan: ninja && ninja check

Reviewers: O1 Bitcoin ABC, #bitcoin_abc, deadalnix

Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D8053