Minor improvements to documentation

Fixed a link, and some formatting.
Bug fix - show more dates & comments in --print-register.

Show date & comment in --print-register for all relevant postings
even if they are not the first posting in the transaction.

Basically, two modes:
* if showing all postings for transactions that affect account
  (--include-related-postings), show date/comment for first posting only.
* if not showing all postings for transactions, show date/comment for
  each posting that affects account.