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ARROW-9390: [C++][Doc] Review compute function names

Modified function names:
* minmax -> min_max
* binary_isascii -> string_isascii (only works on string types)
* ascii_length -> binary_length (also make it work on binary types)
* binary_contains_exact -> match_substring
  (other possibility: has_substring ?)
* match -> index_in
* isin -> is_in
* list_value_lengths -> list_value_length
* partition_indices -> partition_nth_indices
  (other kinds of partitioning would be possible, e.g. using a predicate)

Document string predicate functions (ARROW-9444).

Also fix the allocation of IsValid output buffer in certain cases.

Closes #7755 from pitrou/ARROW-9390-compute-func-names

Authored-by: Antoine Pitrou <antoine@python.org>
Signed-off-by: Antoine Pitrou <antoine@python.org>