RFR: 8343125: Correct the documentation for TreeMap's getFloorEntry and getCeilingEntry [v3]

[Naman Nigam]

> As the documentation of getCeilingEntry currently reads, the inference is misleading as follows:
> 
> if no such entry exists, returns the entry for the least key greater than the specified key;
> if no such entry exists (i.e., the greatest key in the Tree is less than the specified key), returns {@code null}
> Have modified the first section to ensure that its clear that the value returned is upon existence of the key. Added a similar change for getFloorEntry.

Naman Nigam has updated the pull request incrementally with one additional commit since the last revision:

  JDK-8343125 : Empty tree case explicitly specified.

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Changes:
  - all: https://git.openjdk.org/jdk/pull/21729/files
  - new: https://git.openjdk.org/jdk/pull/21729/files/905865a6..48a0fc1a

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=21729&range=02
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=21729&range=01-02

  Stats: 4 lines in 1 file changed: 0 ins; 0 del; 4 mod
  Patch: https://git.openjdk.org/jdk/pull/21729.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/21729/head:pull/21729

PR: https://git.openjdk.org/jdk/pull/21729