Update 'Creating issues using the CSV importer' documentation

I would like to point out some points on Creating issues using the CSV importer documentation that could be improved.

Regarding the "valid" and "invalid" examples:

Those examples need to more clear, stating that the first row must be a header, and subsequent rows will bedata (instead of just two very similar examples, that do not explain clearly why one is correct while the other is not), Additionally, those examples could include the first row (header) and a data row following it.

"The data fields appear to be pretty limited - much moreso than
I'd expect. Some data appears to be a part of my company's specific
structure (i'm relatively new to the company and am still learning), so
things like "HLE" (High-level estimate of work) aren't a default JIRA field."

Regarding Fields:

It would be really great if we could include additional info regarding the fields being exported, as it can clarify each field specific info, such as proposed by a customer below:

"It would be helpful to have a list of fields and their purposes, in order to better prep a CSV import file."

"At a minimum, I would probably reference the base fields and what their
possible values might be."

"For Example:

• Summary: the title of your item to be imported
• Description: The description of your item to be imported
• Issue Type: The index value of the Issue Type (within your project) that this item should be categorized as. This is dependent on your project configuration.
• Assignee: The person who the item will be assigned to. Use their JIRA ID; if you do not, a new user will be created for the ID specified.
• Reporter: The person who is reporting the item in the first place. Again, use their JIRA ID, or a new user will be created for the ID specified."