HADOOP-19667: Clarify constraints for hadoop.security.crypto.buffer.size (min=512; floor to cipher block size)

[zzz0706]

Description of PR

Docs-only change to TransparentEncryption.md clarifying the hidden constraints for
hadoop.security.crypto.buffer.size:

• Minimum value is 512 bytes (values <512 throw IllegalArgumentException at stream construction).
• Block alignment: the effective value is floored to the nearest multiple of the cipher algorithm
  block size (e.g., 16 for AES/CTR/NoPadding). Examples: 4100 -> 4096, 8195 -> 8192.

This is a minimal wording update (single-sentence addition) to avoid operator surprise, with no behavior change.

Target branch: trunk (per contributor guide)

JIRA: HADOOP-19667

Files changed:

• hadoop-hdfs-project/hadoop-hdfs/src/site/markdown/TransparentEncryption.md

How was this patch tested?

• Docs-only; built site/module locally to validate formatting:

[hadoop-yetus]

🎊 +1 overall

Vote	Subsystem	Runtime	Logfile	Comment
+0 🆗	reexec	14m 55s		Docker mode activated.
			_ Prechecks _
+1 💚	dupname	0m 0s		No case conflicting files found.
+0 🆗	codespell	0m 0s		codespell was not available.
+0 🆗	detsecrets	0m 0s		detect-secrets was not available.
+0 🆗	markdownlint	0m 0s		markdownlint was not available.
+1 💚	@author	0m 0s		The patch does not contain any @author tags.
			_ trunk Compile Tests _
+1 💚	mvninstall	40m 25s		trunk passed
+1 💚	mvnsite	1m 19s		trunk passed
+1 💚	shadedclient	77m 7s		branch has no errors when building and testing our client artifacts.
			_ Patch Compile Tests _
+1 💚	mvninstall	1m 4s		the patch passed
+1 💚	blanks	0m 0s		The patch has no blanks issues.
+1 💚	mvnsite	1m 9s		the patch passed
+1 💚	shadedclient	39m 27s		patch has no errors when building and testing our client artifacts.
			_ Other Tests _
+1 💚	asflicense	0m 39s		The patch does not generate ASF License warnings.
		134m 51s

Subsystem	Report/Notes
Docker	ClientAPI=1.51 ServerAPI=1.51 base: https://ci-hadoop.apache.org/job/hadoop-multibranch/job/PR-7907/1/artifact/out/Dockerfile
GITHUB PR	#7907
JIRA Issue	HADOOP-19667
Optional Tests	dupname asflicense mvnsite codespell detsecrets markdownlint
uname	Linux 04003553fdc2 5.15.0-143-generic #153-Ubuntu SMP Fri Jun 13 19:10:45 UTC 2025 x86_64 x86_64 x86_64 GNU/Linux
Build tool	maven
Personality	dev-support/bin/hadoop.sh
git revision	trunk / ce5422f
Max. process+thread count	667 (vs. ulimit of 5500)
modules	C: hadoop-hdfs-project/hadoop-hdfs U: hadoop-hdfs-project/hadoop-hdfs
Console output	https://ci-hadoop.apache.org/job/hadoop-multibranch/job/PR-7907/1/console
versions	git=2.25.1 maven=3.6.3
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