[DOC][MINOR][SPARKR] Update SparkR doc for names, columns and colnames

Update R doc:
1. columns, names and colnames returns a vector of strings, not **list** as in current doc.
2. `colnames<-` does allow the subset assignment, so the length of `value` can be less than the number of columns, e.g., `colnames(df)[1] <- "a"`.

felixcheung

Author: actuaryzhang <actuaryzhang10@gmail.com>

Closes #17115 from actuaryzhang/sparkRMinorDoc.