docs: kdoc: Move content handling into KernelEntry

docs: kdoc: Move content handling into KernelEntry

Rather than having other code mucking around with this bit of internal
state, encapsulate it internally.  Accumulate the description as a list of
strings, joining them at the end, which is a more efficient way of building
the text.

Reviewed-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Link: https://lore.kernel.org/r/20250627184000.132291-3-corbet@lwn.net