Fix doclint warnings in java/nio

There are currently 11 doclint warnings emitted when building API Documentation for java/nio. They are as follows:

./build/linux-x64/support/gensrc/java.base/java/nio/charset/IllegalCharsetNameException.java:47: warning: no comment
    private String charsetName;
                   ^
./open/src/java.base/share/classes/java/nio/charset/MalformedInputException.java:44: warning: no comment
    private int inputLength;
                ^
./open/src/java.base/share/classes/java/nio/charset/UnmappableCharacterException.java:44: warning: no comment
    private int inputLength;
                ^
./build/linux-x64/support/gensrc/java.base/java/nio/charset/UnsupportedCharsetException.java:47: warning: no comment
    private String charsetName;
                   ^
./open/src/java.base/share/classes/java/nio/file/DirectoryIteratorException.java:81: warning: no @param for s
    private void readObject(ObjectInputStream s)
                 ^
./open/src/java.base/share/classes/java/nio/file/DirectoryIteratorException.java:81: warning: no @throws for java.lang.ClassNotFoundException
    private void readObject(ObjectInputStream s)
                 ^
./open/src/java.base/share/classes/java/nio/file/FileSystemException.java:43: warning: no comment
    private final String file;
                         ^
./open/src/java.base/share/classes/java/nio/file/FileSystemException.java:44: warning: no comment
    private final String other;
                         ^
./open/src/java.base/share/classes/java/nio/file/InvalidPathException.java:43: warning: no comment
    private int index;
                ^
./open/src/java.base/share/classes/java/nio/file/InvalidPathException.java:42: warning: no comment
    private String input;
                   ^
./open/src/java.base/share/classes/java/nio/file/attribute/UserPrincipalNotFoundException.java:43: warning: no comment
    private final String name;
                         ^