Add some documentation for the Javadoc patterns.

RELNOTES: Add Javadoc docs.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=215200762

docs/bugpattern/javadoc/InvalidParam.md

@@ -0,0 +1,14 @@
+Javadoc's `@param` tag should not be used to document parameters which do not
+appear in the formal parameter list. This may indicate a typo, or an omission
+when refactoring.
+
+```java {.bad}
+/**
+ * Parses {@code input} as a proto.
+ *
+ * @param inputBytes input bytes to parse.
+ */
+MyProto parse(byte[] input) {
+  ...
+}
+```

docs/bugpattern/javadoc/InvalidTag.md

@@ -0,0 +1,25 @@
+A non-standard Javadoc tag was used, or was used in the wrong way. For example,
+`@param` should be used as a block tag to describe parameters, but cannot be
+used inline to link to parameters (prefer `{@code paramName}` for that).
+
+```java {.bad}
+/**
+ * Doubles {@param n}
+ *
+ * @returns two times n
+ */
+int twoTimes(int n) {
+  return 2 * n;
+}
+```
+
+```java {.good}
+/**
+ * Doubles {@code n}
+ *
+ * @return two times n
+ */
+int twoTimes(int n) {
+  return 2 * n;
+}
+```

docs/bugpattern/javadoc/InvalidThrows.md

@@ -0,0 +1,13 @@
+The `@throws` tag should not document a checked exception which is not actually
+thrown by the documented method.
+
+```java {.bad}
+/**
+ * Validates {@code n}.
+ *
+ * @throws Exception if n is negative.
+ */
+void validate(int n) {
+  ...
+}
+```

docs/bugpattern/javadoc/ReturnFromVoid.md

@@ -0,0 +1,12 @@
+Methods which do not return anything should not have a `@return` tag.
+
+```java {.bad}
+/**
+ * Frobnicates.
+ *
+ * @return a frobnicator
+ */
+void frobnicator(int a) {
+  ...
+}
+```