[doc] use v5.36, only partially enables warnings

[druud]

perl5360delta says:

Furthermore, use v5.36 will also enable warnings as if you'd written use warnings.

but the 'once' warning is an exception:

perl -e'use v5.36; no strict; print $i' 
Use of uninitialized value $i in print at -e line 1.

vs:

perl -e'use v5.36; no strict; use warnings; print $i'
Name "main::i" used only once: possible typo at -e line 1.
Use of uninitialized value $i in print at -e line 1.

[Leont]

I guess this is a side-effect of the optimizer, because adding almost any use line (other than strict?) will make the once warning reappear.

[tonycoz]

Perl_utilize() isn't setting G_WARN_ONCE in PL_dowarn, which triggers the scan for "used only once" symbols after compilation.

I expect the other module you were useing was doing a use warnings; which would set G_WARN_ONCE when warnings.pm set ${^WARNING_BITS}.