indexing: minor edit #2

Author: Bob Grabar

source/faq/indexes.txt

@@ -83,11 +83,10 @@ documentation on choosing which fields to index, see
 How do write operations affect indexes?
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-While a read operation does not affect an index, every write operation
-does involve a write to the index. If your application is write-heavy,
-creating too many indexes might affect performance.
-
-.. todo:: More is needed on that last FAQ.
+While a read operation on an indexed key does not affect indexes, any
+write operation to an indexed key involves writing not only to the
+document but also to the appropriate index. If your application is
+write-heavy, creating too many indexes might affect performance.
 
 Will building a large index affect database performance?
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