Try to make the docs a little better. (#524)

* Try to make the docs a little better.
- Fix stardoc adding <p align=center> in markdown tables.
- Add rough capability to handle macros that wrap a rule.

The wrapping technique is:
- In the wrapping macro, use the docstring  @wraps(some_rule)
- Then we don't emit the docs for the wrapper
- In the docs for some_rule, replace the string "some_rule" with the
  name of the wrapper macro.

It is a first attempt to make things better.  It is crude, but it
mostly works.It is a first attempt to make things better. It is crude, but it mostly works. If Stardoc ever adds this capability [(feature request)[https://github.com/bazelbuild/stardoc/issues/120] we can move to that.