perldiag: Rewrap an entry for better splain output

perldiag: Rewrap an entry for better splain output

In an eighty-column terminal, this is what it looked like before
this commit:

$ ./perl -Ilib -Mdiagnostics -E 'state ($x)=1'
Initialization of state variables in list context currently forbidden at -e line 1, at EOF
Execution of -e aborted due to compilation errors (#1)
    (F) state only permits initializing a single scalar variable, in scalar
    context. So state $a = 42 is allowed, but not state ($a) = 42. To apply
    state semantics to a hash or array, store a hash or array reference in a sca
lar
    variable.