RFR: 8316734: URLEncoder should specify that replacement bytes will be used in case of coding error

[Darragh Clarke]

Currently the descriptions of `URLEncoder.encode` and `URLDecoder.decode` don't specify their use of replacement bytes or replacement character when they cannot handle a character or sequence of bytes. This is longstanding behavior but needs to be documented.

**Solution**
- Added a new line to `URLEncoder.encode` API documentation to document that the charset's replacement bytes are used.

- Also changed `URLDecoder.decode` API documentation to document its use of the charset's replacement character, also changed some wording.

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

Commit messages:
 - specify that replacement bytes will be used in case of coding error

Changes: https://git.openjdk.org/jdk/pull/16709/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=16709&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8316734
  Stats: 12 lines in 2 files changed: 7 ins; 0 del; 5 mod
  Patch: https://git.openjdk.org/jdk/pull/16709.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/16709/head:pull/16709

PR: https://git.openjdk.org/jdk/pull/16709