[lwip-users] docs

[Zschocke, Florian]

The sys_arch.txt file has contradicting entries for the sys_arch_sem_wait()
and sys_arch_mbox_fetch() functions. One says that it is okay to return 0
while the other says it's not.

Maybe you should consider to add a guideline to the contrib.txt file stating
what kind of indenting should be used for the lwIP code. Right now the
indentation of the code is pretty messy. Maybe it would help if some kind of
guideline existed, something in the way of "use only tabs" or "use four
spaces".

Florian.