CLDR-16939 document new unit prefix info (#3591)

Co-authored-by: Steven R. Loomis <[email protected]>

docs/ldml/tr35-info.md

@@ -809,6 +809,44 @@ For example the following values can be suffixes in a simple_unit identifier suc
 <unitIdComponent type="suffix" values="force imperial luminosity mass metric person radius scandinavian troy unit us"/>
 ````
 
+### Unit Prefixes
+```xml
+<!ELEMENT unitPrefixes ( unitPrefix* ) >
+
+<!ELEMENT unitPrefix EMPTY >
+<!ATTLIST unitPrefix type NMTOKEN #REQUIRED >
+<!ATTLIST unitPrefix symbol NMTOKEN #REQUIRED >
+<!ATTLIST unitPrefix power10 NMTOKEN #IMPLIED >
+<!ATTLIST unitPrefix power2 NMTOKEN #IMPLIED >
+```
+
+This data lists the SI prefixes that can be applied to units (typically limited to prefixable units), 
+such as the following:
+```xml
+<unitPrefixes>
+	<unitPrefix type='quecto' symbol='q' power10='-30'/>
+...
+	<unitPrefix type='micro' symbol='μ' power10='-6'/>
+...
+	<unitPrefix type='giga' symbol='G' power10='9'/>
+...
+	<unitPrefix type='quetta' symbol='Q' power10='30'/>
+	<unitPrefix type='kibi' symbol='Ki' power2='10'/>
+...
+	<unitPrefix type='yobi' symbol='Yi' power2='80'/>
+</unitPrefixes>
+```
+The information includes the SI prefix and symbol, and the power of 10 or power of 2 
+(for binary prefixes, intended for use with digital units).
+
+Note that the translated short form of a unit prefix is not the same as the localized symbol. 
+The localized symbol may be the same for most Latin-script languages, 
+but depending on the customary use in a language they can be in a different script
+or use different letters even in Latin-script languages. They are, however, the same in the root locale.
+
+The newer prefixes (quecto-, ronto-, -ronna, -quetta) are not yet being translated, 
+because the appropriate translated versions have not yet been well established across languages.
+
 ### Constants