[Trivial] Add a comment on the use of prevector in script.

Author: gmaxwell

src/script/script.h

@@ -377,6 +377,12 @@ class CScriptNum
     int64_t m_value;
 };
 
+/**
+ * We use a prevector for the script to reduce the considerable memory overhead
+ *  of vectors in cases where they normally contain a small number of small elements.
+ * Tests in October 2015 showed use of this reduced dbcache memory usage by 23%
+ *  and made an initial sync 13% faster.
+ */
 typedef prevector<28, unsigned char> CScriptBase;
 
 /** Serialized script, used inside transaction inputs and outputs */