Update LogicalPlan rustdoc code to not use LogicalPlanBuilder

[andygrove]

Is your feature request related to a problem or challenge? Please describe what you are trying to do.
In #2294 we move the LogicalPlan struct from the datafusion crate to the datafusion-expr crate. Unfortunately, the rustdoc on some methods reference LogicalPlanBuilder, which remains in the datafusion crate, so the rustdoc was tagged with ignore to stop cargo from testing that code.

Describe the solution you'd like
Either update the code to not use LogicalPlanBuilder (or move LogicalPlanBuilder to the datafusion-expr crate, but that may be a large task and require other dependencies to be moved).

Describe alternatives you've considered
None

Additional context
None

[andygrove]

Also need to unignore rustdoc in LogicalPlanBuilder and find a replacement for scan_empty