Explain the need having three validateTargetLocationsXX methods

[Ao-senXiong]

I can not commit this PR so I created a new one.

Merge this PR will close
#582

[Ao-senXiong]

I noticed that there are 13 extra tag: @link errors in JDK 8. The classpath setting seems not enough for JDK 8

[Ao-senXiong]

I did not find a proper fix for the javadoc issue at the comment. Looks like related to classpath for java 8.

checker-framework/build.gradle

Line 349 in 01d2916

classpath += configurations.getByName('checkerFatJar').asFileTree

[Ao-senXiong]

I don't understand this misc_jdk8 failure. I tried fully qualified name for but it still shows the failure.

/__w/1/s/framework/src/main/java/org/checkerframework/common/basetype/BaseTypeVisitor.java:1776: warning - Tag @link: can't find validateWildCardTargetLocation(AnnotatedWildcardType,
 Tree) in org.checkerframework.common.basetype.BaseTypeValidator
/__w/1/s/framework/src/main/java/org/checkerframework/common/basetype/BaseTypeVisitor.java:1689: warning - Tag @link: can't find validateWildCardTargetLocation(AnnotatedWildcardType,
 Tree) in org.checkerframework.common.basetype.BaseTypeValidator