doc: drop unneeded <p> tags

doc: drop unneeded <p> tags

Even before Doxygen gained Markdown support, empty lines were considered
as paragraphs. Changelogs indicate that this was the case since at least
the doxygen 1.2 series (where I found a mentiond that something around
this behaviour was fixed). So at least works in Doxygen versions
released after 2001 [1].

[1] https://sourceforge.net/p/doxygen/mailman/message/172899/

Signed-off-by: Erik Schilling <erik.schilling@linaro.org>
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>