[doc] Add clarity for "qr//" in "The empty pattern //"

[NikitaFeodonit]

Where
https://perldoc.perl.org/perlop#The-empty-pattern-//

Description

Please add to this subsection "The empty pattern //"
https://perldoc.perl.org/perlop#The-empty-pattern-//
for clarity:
"This subsection applies to // and m//, but not to qr//."

We ran into a problem with qr// in 5.14
7e31363
and didn't immediately realize that this subsection of documentation did not belong to qr//.