standardizing docs #6821

standardizing docs #6821
(redo of 2999/I5609025feee8cfdecc09b55bfbf1bd13fa2e6602)

This PR is designed to bring more clarity within the docs by renaming object
instances that may be consfusingly similar to class, method, and attribute names.

For example, instances of the class `MetaData` are available on some  objects as
`.metadata` property, and had appeared within the docs as both `meta` and
`metadata` which has confused some users in the past. By this PR, the docs now
utilize the following naming convention:

* MetaData - SQLAlchemy class
* .metadata - SQLAlchemy API attributes
* metadata_obj - developer instantiated metadata objects or references

Detailed Changes:

* standardized `meta` and `metadata` instances to `metadata_obj`. note: the docs were evenly split between 'meta' and 'metadata'.
* standardized 'cursor' to 'cursor_obj' to avoid confusion with the method.
* standardized a 'scalar_subquery = ' to 'scalar_subq' to avoid confusion with the method.
* standardized a 'cte = ' to 'cte_obj' to avoid confusion with the method

Change-Id: I79c98aee16c5fc6649289b2dd7d6dfc368222fb4