Added doc/UNDOCUMENTED.rst, functions for generating

This collects all known undocumented modules and functions and
puts them in doc/UNDOCUMENTED.rst, using standard function and
method templates so they can be easily filled in and copied to
the correct .rst file in doc/stdlib.

This file can be regenerated with

```julia
cd("$JULIA_HOME/../../doc")
import DocCheck
DocCheck.gen_undocumented_template()
```

#2242 should be updated as functions are documented.