[Rd] make file.rename return invisible

[Dipterix Wang]

Dear R Core devs,

I wonder if it makes sense to make function file.rename return invisible? This is not a big issue, but when running in RMarkdown or knitr, this function will print results. I have to manually call invisible to hide the output from showing in the final document:

```r
invisible(file.rename(...))
```

Otherwise knitr will print:

```
file.rename(...)
#> TRUE
```

If this function is designed for debug/interactive use, and the return is designed to be visible so users don't have to manually print, would it be possible to add an option to suppress the printing? 

Best,
- D
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