[DOC Polish] Column clause doc

[sneaxiy]

The doc of COLUMN clause is not quite clear. For example:

• SELECT a, b, c ... COLUMN a, b . Are the input columns a, b, c or a, b?
• SELECT a, b ... COLUMN b, a. Which is the first feature column, a or b?
• Some column clauses would combine 2 or more selected columns, for example CROSS(col1, col2).
  • SELECT a, b, c COLUMN CROSS(a, c), CROSS(a, b). Do the input columns exclude a, b, c in SELECT clause?
  • SELECT a, b, c, d COLUMN CROSS(b, d). Are the input columns a, CROSS(b, d), c or a, c, CROSS(b, d)?
• COLUMN clauses cannot refer to the old feature name when using AS. For example, SELECT a, b as b_alias ... COLUMN b would raise error.
• It seems that COLUMN ... FOR ... is only used in a few TensorFlow models (for example, DNNLinearCombinedClassifier), and it is not valid in XGBoost models. Is it necessary to add COLUMN ... FOR ... for only these few models? Or, can we find a better way to handle this, instead of adding extra FOR grammar?