Change rules for mark passed to doc:content

Change rules for mark passed to doc:content

The 'start' mark passed to doc:content must not now be changed.
Some handlers did change it, others didn't.
Several callers didn't want it changed (so passed dup), a few didn't
care.

Due to the extra chars that can be passed, it might be awkward to return
a precise mark, and the caller might not care.
So leave it up to the callback to report any marks needed, and always
dup the mark before moving it.

Signed-off-by: NeilBrown <neil@brown.name>