[C++][Doc] Undocumented compute functions (string_isalpha, etc.)

[asfimport]

See #7656 (comment)

Reporter: Antoine Pitrou / @pitrou

Related issues:

• [C++] Review compute function names (is part of)

_{Note: This issue was originally created as ARROW-9444. Please see the migration documentation for further details.}

[asfimport]

Antoine Pitrou / @pitrou:
cc @maartenbreddels

[asfimport]

Maarten Breddels / @maartenbreddels:
Feel free to assign to me, I didn't know there was a place for documenting this. It will however take ~3 weeks before I commit significant time into Arrow again. I hope that's not an issue with 1.0 coming soon.