Documentation to dos

1. As a first pass, actually briefly describe and document all functionality
2. Doxygen is a giant mess. Without preprocessing the source to hell and back not sure what we can do here. https://github.com/foonathan/standardese is interesting but unclear how much of a help that can be; we may still need to either preprocess the source (perhaps only to purgatory and not hell) or give up on the automatic generation.

• CONCEPT_REQUIRES_, CONCEPT_REQUIRES, and (especially) their expanded form should not leak into the signature in the documentation
• What to do about those gigantic trailing return types?
• The global function objects should be documented as if they are function templates, rather than objects

3. Examples. Examples. Examples.