MINOR: [Docs] Update document for AWS_ENDPOINT_URL environment variab…

…le (apache#36826)

### Rationale for this change
update document for AWS_ENDPOINT_URL environment variable

### What changes are included in this PR?
update document for AWS_ENDPOINT_URL environment variable

### Are these changes tested?

### Are there any user-facing changes?
No

Authored-by: yiwei.wang <yiwei.wang@weride.ai>
Signed-off-by: Sutou Kouhei <kou@clear-code.com>

docs/source/cpp/env_vars.rst

@@ -138,6 +138,10 @@ that changing their value later will have an effect.
       SIGILL (Illegal Instruction).  User must rebuild Arrow and PyArrow from
       scratch by setting cmake option ``ARROW_SIMD_LEVEL=NONE``.
 
+.. envvar:: AWS_ENDPOINT_URL
+
+   Endpoint URL used for S3-like storage, for example Minio or s3.scality.
+
 .. envvar:: GANDIVA_CACHE_SIZE
 
    The number of entries to keep in the Gandiva JIT compilation cache.