document doStepIf

docs/buildbot.texinfo

@@ -4848,6 +4848,14 @@ having WARNINGS. The remaining steps will still be executed.
 if True, this build step will always be run, even if a previous buildstep
 with @code{haltOnFailure=True} has failed.
 
+@item doStepIf
+A step can be configured to only run under certain conditions.  To do this, set
+the step's @code{doStepIf} to a boolean value, or to a function that returns a
+boolean value.  If the value or function result is false, then the step will
+return SKIPPED without doing anything.  Oherwise, the step will be executed
+normally.  If you set @code{doStepIf} to a function, that function should
+accept one parameter, which will be the @code{Step} object itself.
+
 @item locks
 a list of Locks (instances of @code{buildbot.locks.SlaveLock} or
 @code{buildbot.locks.MasterLock}) that should be acquired before