HADOOP-17925. BUILDING.txt should not encourage to activate docs profile on building binary artifacts.

[iwasakims]

https://issues.apache.org/jira/browse/HADOOP-17925

If -Pdocs is activated, hadoop-client depends on xerces. The dependency was added by HADOOP-14835 to fix jdiff error. We are avoiding unwanted dependency by building documentations alone in the second pass after building binary artifacts. The BUILDING.txt just should not encourage to activate -Pdocs on building binary artifacts.

[hadoop-yetus]

🎊 +1 overall

Vote	Subsystem	Runtime	Logfile	Comment
+0 🆗	reexec	17m 9s		Docker mode activated.
			_ Prechecks _
+1 💚	dupname	0m 0s		No case conflicting files found.
+0 🆗	codespell	0m 0s		codespell was not available.
+1 💚	@author	0m 0s		The patch does not contain any @author tags.
			_ trunk Compile Tests _
+1 💚	shadedclient	27m 28s		branch has no errors when building and testing our client artifacts.
			_ Patch Compile Tests _
+1 💚	blanks	0m 0s		The patch has no blanks issues.
+1 💚	shadedclient	20m 51s		patch has no errors when building and testing our client artifacts.
			_ Other Tests _
+1 💚	asflicense	0m 29s		The patch does not generate ASF License warnings.
		67m 39s

Subsystem	Report/Notes
Docker	ClientAPI=1.41 ServerAPI=1.41 base: https://ci-hadoop.apache.org/job/hadoop-multibranch/job/PR-3509/1/artifact/out/Dockerfile
GITHUB PR	#3509
Optional Tests	dupname asflicense codespell
uname	Linux a53eb31a50d3 4.15.0-153-generic #160-Ubuntu SMP Thu Jul 29 06:54:29 UTC 2021 x86_64 x86_64 x86_64 GNU/Linux
Build tool	maven
Personality	dev-support/bin/hadoop.sh
git revision	trunk / a10371c
Max. process+thread count	593 (vs. ulimit of 5500)
modules	C: . U: .
Console output	https://ci-hadoop.apache.org/job/hadoop-multibranch/job/PR-3509/1/console
versions	git=2.25.1 maven=3.6.3
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[jojochuang]

+1 make sense to me.

[iwasakims]

Thanks, @jojochuang and @aajisaka. I'm merging this.