Add a berief general comment on BTScanInsertData's nextkey and backward

Hi,

While reading the nbtree codes, I noticed that the comments on
BTScanInsertData no longer describes the meaning of the nextkey and
backward fields. The comment curently only says:

 * See comments in _bt_first for an explanation of the nextkey and backward
 * fields.

Detailed comments used to exit here, but they were removed by
c9c0589fda0e, I guess, because the semantic changed when
the optimazation for backward scans was introduced. However, having
a brief, general description here is still useful for readers.

I've attached a patch that adds the following comment:

+ * nextkey determines how the scankey's boundary is interpreted, and backward
+ * indicates a backward scan.  See comments in _bt_first for a more detailed
+ * explanation of these fields.

What do think?

Regards,
Yugo Nagata

-- 
Yugo Nagata <nagata@sraoss.co.jp>