The key for CouchDB documents cannot begin with an underscore.

This change adds validation in simulation and commit to prevent the
key from beginning with an underscore.

Remove the ~metadata restriction.

Documentation is updated to reflect the change.

Change-Id: I7a0bb9e0af9e513a982806693e64d18fae226d2d
Signed-off-by: Chris Elder <chris.elder@us.ibm.com>