Style exposition-only members of flat_map, flat_multimap, flat_set, flat_multiset consistently

[tkoeppe]

Currently, several private, exposition-only members of the four new "flat" container adaptors are not styled with \exposid. We should review all four class templates and present them according to our usual rules for expostion-only members.

Note that some of these members are classes with their own members. We might want to discuss how much of those should be marked up with \exposid -- only the class name, or also all members? (I think only the class name suffices, since within that class the members are quite definite.)

[tkoeppe]

@Dani-Hub We won't do this in time for the CD, but hopefully soon thereafter!