vswitch.xml: Update docs for max-idle.

vswitch.xml: Update docs for max-idle.

When this configuration parameter was initially introduced into the
database, the documentation was not updated to describe where it
resides. Add the documentation, with the caveat that in most situations,
there is no need to tweak this option and it is primarily present for
the benefit of developers working on flow caching.

Fixes: 72310b041cfa ("upcall: Configure datapath max-idle through ovs-vsctl.")
Reported-by: Hadar Hen Zion <hadarh@dev.mellanox.co.il>
Signed-off-by: Joe Stringer <joestringer@nicira.com>
Acked-by: Ben Pfaff <blp@nicira.com>