[lib] Document why ClientDBMessageInfo uses strings everywhere

Summary: Context: Should close https://linear.app/comm/issue/ENG-237/document-why-clientdbmessageinfo-uses-strings-everywhere

Test Plan: NA

Reviewers: tomek, marcin, varun, O6 Docs, ashoat

Reviewed By: O6 Docs, ashoat

Subscribers: ashoat

Differential Revision: https://phab.comm.dev/D5834