doc: netlink: Fix generated .rst for multi-line docs

doc: netlink: Fix generated .rst for multi-line docs

Fix the newline replacement in ynl-gen-rst.py to put spaces between
concatenated lines. This fixes the broken doc string formatting.

See the dpll docs for an example of broken concatenation:

https://docs.kernel.org/6.9/networking/netlink_spec/dpll.html#lock-status

Signed-off-by: Donald Hunter <donald.hunter@gmail.com>
Link: https://lore.kernel.org/r/20240528140652.9445-2-donald.hunter@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>