Use sv_catpvn instead of sv_catsv in doop.c:do_join

Bunchmarking shows that SvPV+sv_catpvn is faster that sv_catsv.  Why
exactly I don’t know, but perhaps fewer functions and flag checks
are the cause.
mention 'switch' is experimental in feature.pm

The other experimental features already have nice warnings in feature.pm
Rename lvalue references

Also correct the description of lvref magic.  When it was first added,
it was for list assignments only, but that soon changed.
perlref: \@hash{slices} = too