[ostreambuf.iter.cons] Use proper idiom for ostreambuf_iterator constructor effects.

Thanks to FrankHB! Closes #146.
[istreambuf.iterator::proxy] Clean up exposition-only type.

Mark this type as exposition-only in code and remove italics from its members.
[istreambuf.iterator], [ostreambuf.iterator] Remove italics from sbuf_.

sbuf_ is an exposition-only member, but that doesn't mean it needs to
be italicized whenever it's referred to.
[dcl.type.simple]/4 Add missing semicolon in otherwise-bulleted-list.

Closes #133.
[class]/3 Format "identifier" as a grammar term.

Closes #132.