RFR: 8264779: Fix doclint warnings in java/nio [v2]

[Conor Cleary]

> This fix addresses the following warnings which were generated by building JDK API documentation with the `-Xdoclint:all` option enabled:
> 
> ./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;
>                          ^
> Changes to [`genExceptions.sh`](https://github.com/openjdk/jdk/commit/b729d8ed7970737a8a2d4e8aa788df33789faea2) and the two [`exceptions`](https://github.com/openjdk/jdk/commit/b729d8ed7970737a8a2d4e8aa788df33789faea2) templates are to address the warnings concerned with `UnsupportedCharsetException.java` and `IllegalCharsetNameException.java` which are generated when `make jdk-image` is run. A CSR will be filed in due course with respect to these changes.

Conor Cleary has updated the pull request incrementally with one additional commit since the last revision:

  8264779: Simplified comments

-------------

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/3376/files
  - new: https://git.openjdk.java.net/jdk/pull/3376/files/b729d8ed..0b879f15

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=3376&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=3376&range=00-01

  Stats: 2 lines in 2 files changed: 0 ins; 0 del; 2 mod
  Patch: https://git.openjdk.java.net/jdk/pull/3376.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/3376/head:pull/3376

PR: https://git.openjdk.java.net/jdk/pull/3376