Fixed API doc: SRT_INVALID_SOCK instead of SRT_ERROR in str_accept

[lelegard]

The functions srt_accept() and srt_accept_bond() return a SRTSOCKET. In case of error, the returned value is SRT_INVALID_SOCK, not SRT_ERROR. In practice, the two values are identical (-1) but they do not have the same semantics.

[ethouris]

There is #2595 where I'm trying to systematize the use of the right error constant - it's still in draft stage and will have to be refreshed soon, but you have just touched the tip of the iceberg.

The same concerns, for example, the srt_connect function, although this thing is even more complicated. The above mentioned PR, beside adding more constants and types, also defines translation, but then especially for srt_connect it has also introduced a constant designated as 0 returned as it was before because with introduction of groups this function returns also a socket descriptor.

[lelegard]

Good, thanks for the clarification @ethouris.
Sorry for the duplicate, you can drop this one then.

[ethouris]

Not exactly a duplicate. You pointed out actually something important; my PR doesn't include changes in the documentation and I must have missed this thing. I'll update this PR with at least some to do to fix this, too.