How can the illiteracy of the Prophet (peace be upon him) be reconciled with what is stated in Sahih al-Bukhari about his request for writing "a document by which you will never go astray," and his writing of his marriage contract with Aisha (may Allah be pleased with her)?

The Prophet, peace and blessings be upon him, was illiterate; he could not read or write. His Lord described him as such. What has been reported about him writing on the day of Hudaybiyyah is to be understood as him commanding the writing. The Prophet, peace and blessings be upon him, took a ring to seal his letters and messages to kings. Marriage contracts were not written during his time.