RFR: 8247402: Documentation for Map::compute contains confusing implementation requirements

[John Lin]

This is from the mailing list: http://mail.openjdk.java.net/pipermail/core-libs-dev/2020-June/067190.html

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### Progress
- [x] Change must not contain extraneous whitespace
- [x] Commit message must refer to an issue
- [ ] Change must be properly reviewed

### Testing

|     | Linux x64 | Windows x64 | macOS x64 |
| --- | ----- | ----- | ----- |
| Build | ⏳ (3/5 running) | ⏳ (2/2 running) | ⏳ (2/2 running) |



### Download
`$ git fetch https://git.openjdk.java.net/jdk pull/714/head:pull/714`
`$ git checkout pull/714`

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Commit messages:
 - 8247402: Documentation for Map::compute contains confusing implementation requirements

Changes: https://git.openjdk.java.net/jdk/pull/714/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=714&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8247402
  Stats: 11 lines in 1 file changed: 1 ins; 6 del; 4 mod
  Patch: https://git.openjdk.java.net/jdk/pull/714.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/714/head:pull/714

PR: https://git.openjdk.java.net/jdk/pull/714