Missing man pages, incomplete documentation in headers.

[stolk]

This library is missing manual pages.

$ man hid_write
No manual entry for hid_write

Even in the header files, documentation is lacking. The largest omissions:

• No mention whether hid_write() blocks.
• No mention whether hid_close() flushes written data.

[Youw]

Any external help/PRs are welcome here.