[istreambuf.iterator] Join subsections for operations descriptions.

[jensmaurer]

There was one subsection for every operator, yet everything
fits on half a page.

Fixes #1429.

[jensmaurer]

I'll inform Daniel Krügler about the changes for the stable names if this gets approved.

[jensmaurer]

This is a larger issue in all of [predef.iterators]; see #1429 for discussion.

[tkoeppe]

@zygoloid, @jensmaurer: Is this for C++17? It looks like a nice and simple change; can we do this in isolation or would you prefer to only do it together with #1429 in one go?

[jensmaurer]

While this is a larger issue, other situations are less clear-cut than this one. So, I'd be happy to have this merged for C++17. Except that we need to augment the new "stable names migration" annex.

[jensmaurer]

Updated for "stable names migration" annex. Note that "op!=" can't be mentioned, because the exclamation mark badly interacts with LaTeX "glossary" operations. Maybe there's some escape-foo to make it work? @godbyk?

[zygoloid]

A glossary is a weird kind of index, so you probably need to escape the ! with ". I'll give that a go.

[tkoeppe]

@zygoloid: That'd be great, this has been bugging me for a very long time. Please check the .ilg logs to be sure that you've resolved all cases.

[zygoloid]

That works, looking at other makeindex errors now.