In the PGF manual version 3.0.1a (as can be found on CTAN at http://ftp.math.purdue.edu/mirrors/ctan.org/graphics/pgf/base/doc/pgfmanual.pdf ), there is no documentation for the shorten < and shorten > keys. They don't appear in the index, are not hyperlinked, and a simple Ctrl+F hasn't shown anything related in the rest of the PDF.
Regards,
Georges Dupéron
See https://github.com/pgf-tikz/pgf/issues/387