ARROW-8483: [Ruby] Removed irrelevant bits of documentation in Arrow::Table
ARROW-8483: [Ruby] Removed irrelevant bits of documentation in Arrow::Table

These were a copy from example with chunks few lines above:
https://github.com/apache/arrow/blob/master/ruby/red-arrow/lib/arrow/table.rb#L66

Closes #6957 from robertino/master

Authored-by: Robert Borkowski <aa2bb4461bb7726501b94a5f3f095a07579addba@o2.pl>
Signed-off-by: Sutou Kouhei <73218711cdd5d5c30ac66da3cc4d9d9204caf34b@clear-code.com>

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