common/options: document log_to_stderr's conditional default value

common/options: document log_to_stderr's conditional default value

The default value of `log_to_stderr` varies depending on whether Ceph
runs as a daemon or a library. Previously, this was only documented via
the `default` property, which led to confusion when debugging client
applications.

For example, when debugging a CephFS client, setting `debug <subsystem> = 5`
in the configuration file doesn't produce visible debug logs as expected.
This occurs because `common_preinit()` overrides `log_to_stderr` to `false`
when Ceph runs as a library, preventing logs from appearing on stderr.

This commit adds clarification to the `long_desc` field to document this
conditional behavior and help users understand why debug output may not
appear in client scenarios.

Signed-off-by: Kefu Chai <k.chai@proxmox.com>