Defining a TextLoader with `hasHeader=true` and no data sample results in no slot names

[yaeldekel]

Related to issue #2663. We need to document the fact that if we create a TextLoader specifying hasHeader=true but without a data sample, then we are actually just letting the loader know that it needs to skip the first line when it eventually loads from a text file. The output schema of that TextLoader will not contain slot names, since the schema is created when the loader is created, and not when Load is called.