Add documentation about the log file parameter

Fixes #962

Proposed Changes

[ ] CONTRIBUTING guide has been followed.
[ ] PR is based on the current GaNDLF master .
[ ] Non-breaking change (does not break existing functionality): provide as many details as possible for any breaking change.
[ ] Function/class source code documentation added/updated (ensure typing is used to provide type hints, including and not limited to using Optional if a variable has a pre-defined value).
[ ] Code has been blacked for style consistency and linting.
[ ] If applicable, version information has been updated in GANDLF/version.py.
[ ] If adding a git submodule, add to list of exceptions for black styling in pyproject.toml file.
[ ] Usage documentation has been updated, if appropriate.
[ ] Tests added or modified to cover the changes; if coverage is reduced, please give explanation.
[ ] If customized dependency installation is required (i.e., a separate pip install step is needed for PR to be functional), please ensure it is reflected in all the files that control the CI, namely: python-test.yml, and all docker files [1,2,3].
[ ] The logging library is being used and no print statements are left.