Document mitogen environment variables

[moreati]

e.g.

mitogen git:(ansible-2.19) ✗ ag "MITOGEN_" --python mitogen ansible_mitogen 
mitogen/__init__.py
63:_default_profiling = os.environ.get('MITOGEN_PROFILING') is not None

mitogen/utils.py
120:    level = os.environ.get('MITOGEN_LOG_LEVEL', level).upper()

mitogen/profiler.py
116:    env['MITOGEN_PROFILING'] = '1'
117:    env['MITOGEN_PROFILE_FMT'] = os.path.join(tmpdir, fmt)

mitogen/master.py
1403:    profiling = os.environ.get('MITOGEN_PROFILING') is not None

mitogen/core.py
678:    'MITOGEN_PROFILE_FMT',
1332:    MITOGEN_PKG_CONTENT = [
1384:        self._present = {'mitogen': self.MITOGEN_PKG_CONTENT}

ansible_mitogen/services.py
121:    max_interpreters = int(os.getenv('MITOGEN_MAX_INTERPRETERS', '20'))
408:        if os.environ.get('MITOGEN_DUMP_THREAD_STACKS'):

ansible_mitogen/process.py
167:    if os.environ.get('MITOGEN_SAVE_PIDS'):
276:    MuxProcess.profiling = getenv_int('MITOGEN_PROFILING') > 0
286:    Get the multiplexer CPU count from the MITOGEN_CPU_COUNT environment
296:    cpu_count = getenv_int('MITOGEN_CPU_COUNT', default=default)
676:        if 'MITOGEN_ROUTER_DEBUG' in os.environ:
680:        secs = getenv_int('MITOGEN_DUMP_THREAD_STACKS', default=0)
712:            size=getenv_int('MITOGEN_POOL_SIZE', default=32),