Revert "MAINT: change von Mises support to [-pi, pi]"

This reverts commit 024ce0b65b1e1c6c06a2345e13d317cbd5757b1f.
DOC: add a note to the vonmises docstring

Document the fact that von Mises distribution is not designed
to work as a fully-fledged distribution on a segment of a real line.