docs: add warning about potential sensitive data in tracing #820

Job	Run time
filter	1m 16s
build(8) for showcase	10m 44s
build(8) except for gapic-generator-java	4m 27s
unmanaged_dependency_check	9m 15s
showcase-clirr	6m 33s
build(25) except self-service clients	13m 9s
showcase (25)	10m 21s
gapic-generator-java-bom	3m 35s
showcase (11)	10m 41s
showcase (17)	10m 47s
showcase (21)	11m 23s
build(21) except self-service clients	12m 54s
build (11)	14m 38s
build(8) for gapic-generator-java	11m 57s
build (17)	13m 30s
dependency_analyzer_unit_test	1m 19s
	2h 26m 29s

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