perldiag: Document ‘Can't call mro_method_changed_in()’
optree_constants.t: Correct line number in comment

Why we need a reference like this is beyond me, but it is extremely
fragile, because even cosmetic changes to gv.t will throw it off.
That said, at least the next person looking at it won’t be as puzzled
as I was (until the next gv.t change).
[perl #118305] make dtrace sub-entry probe support lexsubs

No tests, because I don’t know how to write them.

See also <https://rt.perl.org/rt3/Ticket/Display.html?id=118305#txn-1221543>.

I have tested this manually, so I know it works and no longer crashes.

Hopefully someone else can follow this up with tests.