Linter `doc_blame` generates unwanted warning when using `old_structure_cmd`

[ocfnash]

The following snippet triggers the "missing doc string" complaint from the linter:

set_option old_structure_cmd true

/-- Foo structure -/
structure foo := (x : ℕ)

/-- Bar structure -/
structure bar extends foo := (y : ℕ)

#lint -- #print bar.to_foo /- def missing doc string -/

This is a minor issue and so far does not seem to have come up very often. @robertylewis gave a helpful summary of the situation here

[bryangingechen]

I ran into this in linear_algebra.basic and I just added a nolint attribute there.

[robertylewis]

As described in #2404, this is a minor issue and not particularly easy to fix, so it's not a priority.