Synopses should have numbered headers

[tkoeppe]

Following PR 560, @zygoloid noticed that there are still several synopses that don't have numbered headers (e.g. strings, algorithms). Uses of \synopsis should be replaced with:

\rSec2[foo.syn]{Header \tcode{<foo>} synopsis}

See also #542.

[tkoeppe]

Question: Should all synopses appear at the top-most level, as a sibling to "General", as we currently do with the containers library? Or is it appropriate to keep them within their respective subsections (and use rSec3)?

For example, consider [diagnostics]. We could either list the synopses of <stdexcept> and <system_error> at the top, right after "General", as the new 19.2, or we could keep them in their current place as 19.2.1 and 19.5.1.

I'm mildly in favour of the latter approach, since we'd otherwise be left with text that wouldn't have a section any more (such as the text after the <system_error> synopsis at the moment).

[tkoeppe]

@tkoeppe: Ping. What should we do with all the remaining \synopsiss?