Document `H5_REQUIRE_IOC`

**Describe the bug**

The following code checks `H5_REQUIRE_IOC`.
```
src/H5FDsubfiling/H5FDsubfiling.c: if ((h5_require_ioc = HDgetenv("H5_REQUIR\
E_IOC")) != NULL) {
```

I don't see any relevant documentation (e.g., txt or md files).

**Expected behavior**
Explain the environment variable somewhere in doc/ or releases_docs/ files.

**Platform (please complete the following information)**
- HDF5 version: develop

**Additional context**
n/a