Fix errors preventing javadoc generation on Java 8

Javadoc generation using Java 8 is now much more strict by default than it was in earlier Java versions, which means that javadocs fail to be generated when executing **mvn javadoc:aggregate** using a Java 8 JDK. For example:

    [ERROR] Failed to execute goal org.apache.maven.plugins:maven-javadoc-plugin:2.9.1:aggregate (default-cli) on project jzlib: An error has occurred in JavaDocs report generation:
    [ERROR] Exit code: 1 - /home/mbooth/git/jzlib/src/main/java/com/jcraft/jzlib/ZStream.java:375: warning: no @return
    [ERROR] public int end(){ return Z_OK; }
    [ERROR] ^
    [ERROR] /home/mbooth/git/jzlib/src/main/java/com/jcraft/jzlib/GZIPHeader.java:40: error: unexpected content
    [ERROR] * @see "http://www.ietf.org/rfc/rfc1952.txt"
    [ERROR] ^
    [ERROR] 
    [ERROR] Command line was: /usr/lib/jvm/java-1.8.0-openjdk-1.8.0.5.x86_64/jre/../bin/javadoc @options @packages

The change in this pull request fixes the minor errors in the javadoc comments and enables javadoc generation to succeed under Java 8.