Document the semantics of `OpenMode`

First mentioned here: https://github.com/input-output-hk/fs-sim/pull/85#discussion_r1895619049

The semantics of `OpenMode` are not properly documented anywhere. For example, opening a file in read mode *should*/*will* throw an error if the file does not exist, but this is not documented. More importantly, it is not documented whether files can be opened multiple times in different modes: for example, can a file be opened in both read- and write-mode?

Start with the OpenMode definition and the linked pull request discussion to determine the existing behavior. Document whether read mode fails for missing files and whether a file can be opened multiple times in different modes, including the relevant examples and expected outcomes.

Written by the indexing model from the issue text.