Merge pull request #116 from takikawa/clarify-segment-semantics

Add note with explicit purpose of 1, 4, and 5 field segments

source-map.bs

@@ -250,10 +250,14 @@ The fields in each segment are:
 Note: The purpose of this encoding is to reduce the source map size. VLQ encoding reduced source maps by 50% relative to the [[V2Format]] in tests performed
 using Google Calendar.
 
+Note: Segments with one field are intended to represent generated code that is unmapped because there
+is no corresponding original source code, such as code that is generated by a compiler. Segments
+with four fields represent mapped code where a corresponding name does not exist. Segments with five
+fields represent mapped code that also has a mapped name.
+
 Note: Using file offsets was considered but rejected in favor of using line/column data to avoid becoming
 misaligned with the original due to platform-specific line endings. 
 
-
 Resolving Sources {#resolving-sources}
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