improve names docstring (#3178)

Author: bkamins

src/abstractdataframe/abstractdataframe.jl

@@ -39,7 +39,9 @@ Return a freshly allocated `Vector{String}` of names of columns contained in `df
 If `cols` is passed then restrict returned column names to those matching the
 selector (this is useful in particular with regular expressions, `Cols`, `Not`, and `Between`).
 `cols` can be:
-* any column selector ($COLUMNINDEX_STR; $MULTICOLUMNINDEX_STR)
+* any column selector ($COLUMNINDEX_STR; $MULTICOLUMNINDEX_STR); these column
+  selectors are documented in the [General rules](@ref) section of the [Indexing](@ref)
+  part of the DataFrames.jl manual
 * a `Type`, in which case names of columns whose `eltype` is a subtype of `T`
   are returned
 * a `Function` predicate taking the column name as a string and returning `true`