TCP_NOTSENT_LOWAT option

`TCP_NOTSENT_LOWAT` is not documented in man pages, but the documentation is available in the [commit](https://github.com/torvalds/linux/commit/c9bee3b7fdecb0c1d070c7b54113b3bdfb9a3d36) which introduced it:

> Idea of this patch is to add optional limitation of number of
unsent bytes in TCP sockets, to reduce usage of kernel memory.
>
> TCP receiver might announce a big window, and TCP sender autotuning
might allow a large amount of bytes in write queue, but this has little
performance impact if a large part of this buffering is wasted

Start by reading the linked Linux commit to understand the TCP_NOTSENT_LOWAT option and identify the relevant man-page source in this repository. Update the appropriate networking documentation so the option and its purpose are described, then build or check the man pages to confirm the new entry is included.

Written by the indexing model from the issue text.