Update low-level backup documentation to match actual behavior

As discussed in [1] our low-level backup documentation does not quite
match the actual behavior of the functions on primary vs. standby.
Since it appears we have decided that the remaining behavioral
differences after 52f8a59dd953c68 are bugs in the documentation, the
attached is a first pass at bringing the documentation up to date.

The biggest change is to recognize that exclusive backups can only be
run on a primary and to adjust the text accordingly. Also, I did not
mention the wait_for_archive param in the exclusive instructions since
they are deprecated.

This patch should be sufficient for 10/11 but will need some minor
changes for 9.6 to remove the reference to wait_for_archive. Note that
this patch ignores Michael's patch [2] to create WAL history files on a
standby as this will likely only be applied to master.

In addition, I have formatted the text to produce minimal diffs for
review, but it could be tightened up before commit.

--
-David
david(at)pgmasters(dot)net

[1]
https://www.postgresql.org/message-id/20170814152816.GF4628%40tamriel.snowman.net
[2]
https://www.postgresql.org/message-id/CAB7nPqQvVpMsqJExSVXHUwpXFRwojsb-jb4BYnxEQbjJLfw-yQ%40mail.gmail.com