Wrong doc link. Correct one:
http://www.fast-downward.org/OptionSyntax#Conditional_options

Merged and documented on http://www.fast-downward.org/PlannerUsage.

Code available on https://bitbucket.org/malte/downward/commits/issue448.
If this is merged, the feature needs to be documented on the wiki.

This is about adding options to the search component that allow specifying
different behaviour for unit-cost and and non-unit-cost problems. Currently, we
only support this for the LAMA 2011 configuration via some special-casing in the
src/search/downward shell script. By adding the functionality directly to the
search component, we can simplify and generalize other code.

See point 2. in msg3259.