Improve docstings of OpenEphysBinaryRawIO

[h-mayorquin]

While working on fixing a neuroconv issue, I noticed that the OpenEphysBinaryRawIO class lacks clear documentation about how the folder structure of the Open Ephys binary format is parsed and use internally.

This PR improves the situation in two ways:

1. The docstring now more explicitly describes the type of structure being parsed and clarifies that dirname can point to different levels within the folder tree, each yielding different parsing results.

2. The current explore_folder function mixes two responsibilities:

   • Parsing the folder structure.
   • Mapping that structure into Neo.

   To separate concerns, I split this logic into two private functions. Each now has a dedicated docstring that clearly explains its role. There is no change in implementation the function is just split in two.

[zm711]

I think this looks pretty great. The diff is huge since the functions are split so it'll take a moment to review :)

[alejoe91]

Nice rewrite @h-mayorquin! This added docs will make it much easier to maintain for us too! :)