[Rd] Rd for write.table {utils}: Be explicit about the default separator

[Timothy Bates]

In "utils/write.table.Rd" the default separator is shown as the invisible character "	". 

In the further documentation, the default separator is not named.

It would helpful to specify it using an escape character in the \Usage section "\s" or "\t"?

and to name it explicitly in the \Arguments section