doc: make error descriptions more concise

Remove the practice of starting most error descriptions with "Used when"
or wordier variations.

Change errors of the form:

> Used when the type of an asynchronous resource is invalid.

...to:

> The type of an asynchronous resource was invalid.

Change errors of the form:

> The `'ERR_INVALID_CURSOR_POS'` is thrown specifically when a cursor on
> a given stream is attempted to move to a specified row without a
> specified column.

...to:

> A cursor on a given stream cannot be moved to a specified row without
> a specified column.

PR-URL: nodejs#16954
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>
Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>