docs(spanner): update JDBC read_lock_mode documentation #18957

Job	Run time
package_name_check	1m 5s
gapic-bom	1m 44s
owlbot-yaml	1m 4s
gitignore_correctness	1m 9s
group_id_check_for_maps_libraries	9m 13s
generate-readme	1m 12s
root-pom	1m 41s
owlbot-py	1m 10s
	18m 18s

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