Describe BAI changes for 'samtools idxstats' and details of BIN value

(Original commit 16 Mar 2012, rebased 8 April 2014)
Add n_no_coor to index formats given that it has been being written by samtools for a few years.
Move "optional" to the first column

Also regularise \cline and fix tabix header typo.
Describe pseudo-bins in a separate table

Hopefully makes clearer that the pseudo-bin is just another bin -- in
fact it doesn't even need to be the final bin in the list!  Add footnote
explaining placed/unplaced.
Merge BAI/CSI/tabix idxstats details (PR #2)