Add documentation to method checkLazySingletonInstanceExists

[deandreamatias]

I think it might be optimal to add documentation (dart doc comments) to the method checkLazySingletonInstanceExists.

Is a useful method, as this issue demonstrates.
Although it may seem self-explanatory due to the name, I would add that it is recommended to use it in conjunction with the isRegistered method, to avoid unexpected problems.

[escamoteur]

would you be willing to write a comment so that you would think its good explained?

[escamoteur]

I asked cursor and it did a good job :-)

[escamoteur]

Included in 8.1.0

[deandreamatias]

Thanks for update!