Clarify docs for Options.base_manager_name.

[Anton Gilgur]

There is currently no documentation in topics or the reference about how specifically to use Meta.base_manager_name and I think an example could alleviate the confusion this causes.

I had some trouble in a library I maintain (​https://github.com/agilgur5/django-serializable-model/issues/4#issuecomment-519338032), where I mistakenly thought that base_manager_name should be set to, literally the name of the manager, as in the string equivalent to the name of the class of the manager (in my case, 'SerializableManager'), similar to how one may reference models by string.
Through finding some examples online like ​https://stackoverflow.com/a/48124863/3431180, I realized it's supposed to be the name of the attribute on the current class that contains the instance of the manager (in my case, 'objects').

An example would really clear things up very concisely.

[Carlton Gibson]

Thanks for this! Yes, but as per comment on PR, I think a clarification of the ​Options.base_manager_name description is what's needed.

Currently, "The name of the manager..." is ambiguous as you say. Narrowing that to "The attribute name...", adding perhaps "... for example 'objects'..." if we think that's clearer should be enough.

[Carlton Gibson <carlton.gibson@…>]

In 514efa31:

Fixed #30688 -- Clarified base_manager_name docs.

[Carlton Gibson <carlton.gibson@…>]

In c5cbb797:

[2.2.x] Fixed #30688 -- Clarified base_manager_name docs.

Backport of 514efa3129792ec2abb2444f3e7aeb3f21a38386 from master