doc: sockets: Update to the current state of affairs
doc: sockets: Update to the current state of affairs

1. There's now proper fdtable, pointer casting to "fd" is gone.
2. select() was implemented.
3. Enumerate implemented functions more thoroughly, so users can
fulltext-search them.

Signed-off-by: Paul Sokolovsky <35cb557f011771c94f127d6fef99f804b62203f5@linaro.org>

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