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

[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.

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Commit messages:
 - JDK-8343125 : Correct the documentation for TreeMap's getFloorEntry and getCeilingEntry
 - Correction: Documentation update for ceil and floor

Changes: https://git.openjdk.org/jdk/pull/21729/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=21729&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8343125
  Stats: 10 lines in 1 file changed: 2 ins; 2 del; 6 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