gh-68335: Documenting csv dialect defaults

[mehrdadmoradii]

Adding documentation for the default dialect used by csv- starting from where #25989 (comment) left off.

• Issue: Dialect class defaults are not documented. #68335

[terryjreedy]

Dialects and Formatting Parameters section needs some work also.

EDIT: The reason the new text seems out of place (see above and resolved comment) is because it is. The original author,Brandon Milam ( jbmilam) of csv_dialect_doc_clarify.patch: "Document clarification" put it at the end of the first paragraph of the Dialects and Formatting Parameters section, as said in #68335 (comment), not at the end of the first paragraph of the class Dialect entry.

I am closing this and starting over.

[bedevere-app]

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