To control trimming doctest flags manually, this adds new options
:trim-doctest-flags: and :no-trim-doctest-flags: to doctest
directives.  It helps to describes doctest module itself in python
doc (see #6698).

To allow to give (or not to give) an explicit title to the
pending_xref nodes on translation, this skips to override the
attribute on merge original and translated documents.

Also extend documentation of 'missing-reference'.