Doc: small update for scaling explanation

Signed-off-by: Peter Sabaini <[email protected]>

docs/explanation/cluster-scaling.rst

@@ -67,7 +67,7 @@ safety checks.
 
 Custom Crush Rules
 ++++++++++++++++++
-Please note, users can freely set custom CRUSH rules anytime. MicroCeph will respect custom rules and not perform any automatic updates for these. Custom CRUSH rules can be useful to implement larger failure domains such as rack- or room-level. At the other end of the spectrum, custom CRUSH rules could be used to enforce OSD-level failure domains for clusters larger than 3 nodes.
+MicroCeph automatically manages two rules, named `microceph_auto_osd` and `microceph_auto_host` respectively; these two rules must not be changed. Users can however freely set custom CRUSH rules anytime. MicroCeph will respect custom rules and not perform any automatic updates for these. Custom CRUSH rules can be useful to implement larger failure domains such as rack- or room-level. At the other end of the spectrum, custom CRUSH rules could be used to enforce OSD-level failure domains for clusters larger than 3 nodes. 
 
 
 Machine Sizing