install option ignored

Got an invalid option error while installing asciidoctor-pdf on Windows 7 x64.

gem install asciidoctor-pdf --pre
Fetching: asciidoctor-pdf-1.5.0.alpha.16.gem (100%)
Successfully installed asciidoctor-pdf-1.5.0.alpha.16
invalid options: -ri
(invalid options are ignored)
Parsing documentation for asciidoctor-pdf-1.5.0.alpha.16
Installing ri documentation for asciidoctor-pdf-1.5.0.alpha.16
Done installing documentation for asciidoctor-pdf after 0 seconds

Be concerned, or not?

I've got the same message doing a clean installation on Windows 7, Ruby 2.4 (ruby 2.4.1p111 (2017-03-22 revision 58053) [x64-mingw32]).
I am also wondering if this is an issue or not. The pdf generation worked fine though.