runtime(doc): clarify documentation for "v" position at line()

runtime(doc): clarify documentation for "v" position at line()

Problem: the previous documentation falsely states that "v" always
refers to the start of a visual area.  In fact, the reference of "v" and
"." complement each other.  If the cursor is at the start of
a (characterwise) visual area, then "v" refers to the end of the area.

Solution: be more verbose and explicit about the connection between "."
and "v" and also refer to |v_o| which many vim users will be familiar
with for visual areas.

Signed-off-by: Peter Aronoff <peter@aronoff.org>
Signed-off-by: Christian Brabandt <cb@256bit.org>