[PATCH] Document the order of changing certain settings when using hot-standby servers

The attached patch updates the hot-standby documentation (in the high
availability section) so it explicitly mentions that certain settings
need to be applied to servers in a particular order. For example, it
states that if you increase a certain setting (e.g. max_connections) you
need to do so on a primary first, before applying it to any secondaries.

Previously this was not explicitly mentioned and could lead to one
thinking they can just apply the setting to all servers in parallel,
resulting in standby servers refusing to start.

The exact phrasing currently used in the patch may be a bit rough, I'm
open to suggestions on how to best improve the writing.

The patch is based on the master branch and applies cleanly to it.

Yorick