Document behavior of implicit loading of mechanisms

[JCGoran]

When running import neuron from Python, or nrniv directly, NEURON implicitly loads any additional mechanisms from the ${ARCH} subdirectory of the current working directory. At least in the case of Python, this can be traced to the load_mechanisms function:

nrn/share/lib/python/neuron/__init__.py

Line 300 in 90900fa

def load_mechanisms(path, warn_if_already_loaded=True):

We should clarify how this works in the online documentation, notably:

• the default case when there is an ${ARCH} subdir in the current working dir
• the case when NRN_NMODL_PATH env variable is set
• any other case of interest