Comma-separated base_type list (#3210)

Author: ralfhandl

registries/_format/decimal.md

@@ -2,7 +2,7 @@
 owner: baywet
 issue: 889
 description: A fixed point decimal number of unspecified precision and range
-base_type: string number
+base_type: string, number
 layout: default
 remarks: This format is used in a variety of conflicting ways and is not interoperable.
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