Add API documentation pre-commit hook.

Use an `if` here. Variable should be quoted and the grep strings look better if they're quoted too.

```shell
if grep 'json:' "${FILE}" | grep -v ',inline' | grep -v -q 'description:' ; then
  echo "0"
else
  echo "1"
fi
exit 0
```

Why a separate script instead of a shell function elsewhere?

And why print "0" or "1" instead of a more traditional interface like returning 0 or non-zero?

This is really confusing btw:

```shell
# Print 1 if the file in $1 is not in need of additional field descriptions, 0 otherwise.
```

Why not call this script `need-field-descriptions.sh` or similar and have it return 0 if they are needed?