Discourage MainUser

Puts a comment, warning people to avoid using the main user concept.
Features should preferably work for all users, or all admin users, or
some other intelligently selected user(s). The idea of having one fixed
special user is against the long-term goals of multiuser.

Bug: 390264303
Test: N/A - just documentation
Flag: EXEMPT documentation only
Change-Id: Iaf56cb7f4c9c1551578e257e950f021b7f6c7163