Add missing chemical-formula

Added missing chemical-formula which was listed in the chemical-element example. Moved that example to chemical-formula.

Added comment to 'chemical-element' property for clarification.

Author: NSoiffer

_data/core-properties.yml

@@ -320,6 +320,10 @@
       en: rectangle A B C D
 
 - property: chemical-element
+  type: chemistry
+  comment: typically added to an `mi` to indicate this is a chemical element (see `chemical-formula` for an example
+  
+- property: chemical-formula
   type: chemistry
   examples:
     - mathml:
@@ -333,7 +337,6 @@
         </mrow>"
       en: cap h 2 cap o
 
-
 - property: chemical-equation
   type: chemistry
   effect: read the term as a chemical equation