[doc] perldebguts: lvalue subroutine and &DB::lsub

[KES777]

Where
https://metacpan.org/pod/perldebguts#Debugger-Internals

If the call is to an lvalue subroutine, and &DB::lsub is defined &DB::lsub(args) is called instead, otherwise falling back to &DB::sub(args).

Description
when my debugger defines DB::sub and does not DB::lsub the next error occur:

Can't modify non-lvalue subroutine call of &DB::sub at

So if my debugger does not define DB::lsub subroutine then this is impossible to debug scripts with lvalue subroutines.

Probably perl should not try to debug lvalue subroutine call if required DB::lsub is not defined.
Just like debugging does not occur for ordinal subs if DB::sub is not defined

Or we should fix otherwise falling back to &DB::sub(args). part of the sentence, because falling back to DB::sub sub for DB::lsub is meaningless because of error

perl v5.24.1

[KES777]

probably related to #13264.
DB::lsub should not relay on DB::sub, because theirs behavior are different.

[KES777]

as workaround I define lsub subroutine: sub DB::lsub : lvalue { &$DB::sub }