[C++] Document and rename is_in_meta_binary

[asfimport]

The is_in_meta_binary and index_in_meta_binary functions do not have any "_doc" elements. I had simply ignored them assuming they were some kind of specialized function that shouldn't be exposed for general consumption (see ARROW-13949) but I recently discovered they are legitimate binary variants of their unary counterparts.

If we want to continue to expose these functions we should rename them (meta I assume means meta function but the python/r user has no idea what a meta function is) and add _doc elements.

Reporter: Weston Pace / @westonpace

Related issues:

• [C++] The index_in_meta_binary/is_in_meta_binary kernels are missing a docstring (is related to)

_{Note: This issue was originally created as ARROW-14262. Please see the migration documentation for further details.}

[asfimport]

Weston Pace / @westonpace:
Also, we will need to add them to compute.rst

[asfimport]

Antoine Pitrou / @pitrou:
The docstrings have been added in ARROW-14751 but the rest of this issue still holds.