[ofiwg] input on which error codes to pass through 'sread' calls

[Hefty, Sean]

> This query is in response to the following pull request:
> 
> https://github.com/ofiwg/libfabric/pull/4829

Based on the PR and private exchanges, I'm proposing adding clarifications to the man page:

https://github.com/ofiwg/libfabric/pull/4838

This documents that EINTR would convert to EAGAIN, though in less specific terms.

- Sean