ARROW-13388: [C++][Parquet] Fix documentation to reflect the reading support for DELTA_LENGTH_BYTE_ARRAY (#13530)
ARROW-13388: [C++][Parquet] Fix documentation to reflect the reading support for DELTA_LENGTH_BYTE_ARRAY (#13530)

Authored-by: Muthunagappan Muthuraman <406b910189744e3237edbf9ccdf10788faced6b0@snowflake.com>
Signed-off-by: Sutou Kouhei <73218711cdd5d5c30ac66da3cc4d9d9204caf34b@clear-code.com>
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