manual and man page do not explain that stderr is used

[gdt]

Instructions

I have thoroughly read the Reporting Bugs and Feature Requests wiki page.

Environment

• reading sources at git master

Expected behavior vs actual behavior

As reported on unison-users@, unison uses stderr for meta information. While this is arguably consistent with Unix tradition, it's arguable enough that it should be documented.

This ticket asks that the manual and man page be changed to state that version/starting information and sync questions appear on stderr, after confirming via code inspection that this is entirely true.