Update inline documentation for WPDB

[theMikeD]

Having accurate documentation is critical for developers, especially for complex and core functionality such as the wpdb class. We on the documentation team have been working to update the docs, and it is now wpdb's turn to be reviewed.

The wpdb page in the official docs includes parts taken from the code, some of which is either formatted incorrectly, misleading, or incorrect. This ticket will track the update to the code to fix the inline docs and docblocks so that its documentation(​https://developer.wordpress.org/reference/classes/wpdb/) is accurate.

The changes include:

• proper spacing (eg. *@since 3.4.0 vs * @since 3.4.0
• proper order of docblock elements (see ​https://make.wordpress.org/core/handbook/best-practices/inline-documentation-standards/php/#1-functions-class-methods)
• consistent language (eg. 'last' vs 'previous')
• use of 80char width
• proper punctuation (eg. all comments end with a period)
• in some cases, editing for clarity (specifically the class description)

Note that there are only comment changes in this patch.

[theMikeD]

Gentle nudge. Can this get into 5.5?

[johnbillion]

In 47740:

Docs: Update inline documentation for the wpdb class.

Props theMikeD

Fixes #49477

[johnbillion]

Thanks for these updates @theMikeD! All looking good.

I made one change which is to remove the vertical alignment of the @return statements because this isn't something that the core inline documentation standard includes.