📝 Clarify how to use 'observed usage'

Author: shnizzedy

docs/_sources/user/compute_config.rst

@@ -10,7 +10,7 @@ Computer Settings
 
 #. **Raise Insufficient - [True, False]:** If True (default), C-PAC will raise an error before trying to run a pipeline if it estimates the given data and pipeline configuration will exceed the memory or core limits. If False, C-PAC will raise a warning and continue to run the pipeline even if it estimates the given data and pipeline configuration will exceed the memory or core limits.
 
-#. **Observed Usage**
+#. **Observed Usage:** The resource usage of each node depends on many factors, including the data, the pipeline configuration, and the system. To run many subjects with optimimzed resource estimation, first run a single subject with the desired number of cores and with a generous memory limit. Then, provide the ``callback.log`` generated from that initial run when you run the rest of the subjects.
     #. **Callback log - [text]:** The path to a callback log file from a previous run, including any resource-management parameters that will be applied in this run, like ``n_cpus`` and ``num_ants_threads``. This file is used override memory estimates with previously observed memory usage. Can be overridden with the commandline flag ``--runtime_usage``.
     #. **Buffer - [percent]:** A percent of the previously observed memory usage that is to be added to the memory estimate. Default: 10. Can be overridden with the commandline flag ``--runtime_buffer``.