[java] UseExplicitTypes - improve rule documentation

pmd-java/src/main/resources/category/java/bestpractices.xml

@@ -1696,8 +1696,21 @@ public class Foo {
           class="net.sourceforge.pmd.lang.rule.XPathRule"
           externalInfoUrl="${pmd.website.baseurl}/pmd_rules_java_bestpractices.html#useexplicittypes">
         <description>
-Java 10 introduced the `var` keyword. This reduces the amount of typing but decreases the reading comprehension of the
-code.
+Java 10 introduced the `var` keyword. This reduces the amount of code written because java can infer the type
+from the initializer of the variable declaration.
+
+This is essentially a trade-off: On the one hand, it can make code more readable by eliminating redundant
+information. On the other hand, it can make code less readable by eliding useful information. There is no
+blanket rule for when `var` should be used or shouldn't.
+
+It may make sense to use `var` when the type is inherently clear upon reading the statement
+(ie: assignment to either a literal value or a constructor call). Those use cases
+can be enabled through properties.
+
+Notice that lambda parameters are allowed, as they are already inferred  by default (the `var` keyword
+is completely optional).
+
+See also [Local Variable Type Inference Style Guidelines](https://openjdk.org/projects/amber/guides/lvti-style-guide).
         </description>
         <priority>3</priority>
         <properties>