Java: Add change note.

Author: aschackmull

change-notes/1.21/analysis-java.md

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+# Improvements to Java analysis
+
+## New queries
+
+| **Query**                   | **Tags**  | **Purpose**                                                        |
+|-----------------------------|-----------|--------------------------------------------------------------------|
+
+## Changes to existing queries
+
+| **Query**                  | **Expected impact**    | **Change**                                                       |
+|----------------------------|------------------------|------------------------------------------------------------------|
+
+## Changes to QL libraries
+
+* The `Guards` library has been extended to account for method calls that check
+  conditions by conditionally throwing an exception. This includes the
+  `checkArgument` and `checkState` methods in
+  `com.google.common.base.Preconditions`, the `isTrue` and `validState` methods
+  in `org.apache.commons.lang3.Validate`, as well as any similar custom
+  methods. This means that more guards are recognized yielding precision
+  improvements in a number of queries including `java/index-out-of-bounds`,
+  `java/dereferenced-value-may-be-null`, and `java/useless-null-check`.
+
+