docs: improve VM compatibility justification

Co-authored-by: Ashley Cliff <[email protected]>

Author: dsloanm

reference/compatibility-matrix.md

@@ -1,7 +1,9 @@
 (reference-compatibility-matrix)=
 # Compatibility Matrix
 
-The tables below provide an overview of cloud instance types that have been tested for compatibility with Charmed HPC revisions. Key:
+The tables below provide an overview of cloud instance types that have been tested for compatibility with Charmed HPC revisions. Inclusion in a table indicates that the corresponding VM type has been tested; however, exclusion from the table simply indicates that the instance has not been tested thoroughly and may be compatible or incompatible with Charmed HPC. 
+
+The compatibility key:
 
 * <span style="color:green">&#x2714; Compatible</span>: Fully compatible with all Charmed HPC features. No known issues.
 * <span style="color:orange">&#x25B3; Partial</span>: May be compatible with a subset of Charmed HPC features or have issues that can be worked around to allow for basic use.