have an actual `NullColumn` object and update docs accordingly.

Follow up to #15438 and #15502.

/cc @sgrif
Emit suggested generator names when not found

When someone types in a generator command it currently outputs all generators. Instead we can attempt to find a subtle mis-spelling by running all generator names through a levenshtein_distance algorithm provided by rubygems. 

So now a failure looks like this:

```ruby
$ rails generate migratioooons
Could not find generator 'migratioooons'. Maybe you meant 'migration' or 'integration_test' or 'generator'
Run `rails generate --help` for more options.
```

If the suggestions are bad we leave the user with the hint to run `rails generate --help` to see all commands.
[ci skip] remove double "in your"
Merge pull request #15497 from schneems/schneems/levenshtein-generator-failure-messages

Emit suggested generator names when not found
Removed CaptureHelper#flush_output_buffer as it is only used in tests.
reviewed: @tenderlove
Fix inverse associations test

`InverseHasManyTests#test_parent_instance_should_be_shared_within_create_block_of_new_child`
was mistakenly the same as
`InverseHasManyTests#test_parent_instance_should_be_shared_within_build_block_of_new_child`.
Update getting_started.md

It may be confusing to a Rails newbie why "Article" is suddenly being referred to with the "A" being capitalized. Thus far in the guide, the "a" has always been in lowercase. I think it is helpful to point out the reason for this change (especially since not using the capital "A" will result in an error). [ci skip]
Update getting_started.md

It's not very clear exactly what the "params" method is and does, as the current description in the guide is somewhat vague and brief. An example is helpful, especially considering this method is referenced over 30 times in this guide. [ci skip]
Merge pull request #15515 from leafac/fix-inverse-associations-block-test

Fix inverse associations test
Merge pull request #15518 from jonathancadepowers/master

Added Two Clarification "Tips" [ci skip]
Merge pull request #15421 from gchan/time_with_zone_precision

Fixed `ActiveSupport::TimeWithZone#-` so precision is not unnecessarily lost
Remove deprecated method ActiveRecord::Migrator.proper_table_name
Merge pull request #15512 from akshay-vishnoi/delete-deprecated-method

Remove deprecated method ActiveRecord::Migrator.proper_table_name
Fix behavior of handling BC era dates.

BC era year is (astronomical year + 1) and starts from 1 BC.
document include_blank's usage with a string [ci skip]
Avoid hardcoded value in test setup/teardown.
Merge pull request #15525 from zuhao/refactor_actionpack_query_string_parsing_test

Avoid hardcoded value in test setup/teardown.
Mime::PNG is already defined.
Merge pull request #15521 from edogawaconan/fix_bc_postgres_master

Fix BC year handling in postgres
Merge pull request #15529 from zuhao/refactor_actionpack_mime_type_test

Make sure Mime::Type is not altered after tests.