An ant property to skip building docs

It can be situationally useful to run:

    ant -Dskip.docs=1 dist-maven

When troubleshooting problems that require creation of
a distribution, such as pr-integrate-partest.

Scaladoc isn't incremental, so you burn
a few minutes for a single file change to the compiler or
library.
Merge pull request #3031 from retronym/topic/skip-doc

An ant property to skip building docs