doc: document the caveat concerning line lookups

doc: document the caveat concerning line lookups

GPIO line names in the kernel aren't unique - neither globally nor
within a single chip. The current API doesn't take this into
consideration and simply returns the first matching line in
gpiod_line_find() and co.

This will be addressed in v2.0 but we can't fix it in v1.x without
introducing new interfaces and deprecating the ones that already exist.

Let's document this behavior in doxygen comments for all affected
routines.

Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>