docs: kdoc_item: fix a typo on sections_start_lines

docs: kdoc_item: fix a typo on sections_start_lines

Currently, there are 15 occurrences of section?_start_lines,
with 10 using the plural way.

This is an issue, as, while kdoc_output works with KdocItem,
the term doesn't match its init value.

The variable sections_start_lines stores multiple sections,
so placing it in plural is its correct way.

So, ensure that, on all parts of kdoc, this will be referred
as sections_start_lines.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Message-ID: <d1e0f1d3f80df41c11a1bbde6a12fd9468bc3813.1773823995.git.mchehab+huawei@kernel.org>