bpftool: Fix documentation about line info display for prog dumps

bpftool: Fix documentation about line info display for prog dumps

The documentation states that when line_info is available when dumping a
program, the source line will be displayed "by default". There is no
notion of "default" here: the line is always displayed if available,
there is no way currently to turn it off.

In the next sentence, the documentation states that if "linum" is used
on the command line, the relevant filename, line, and column will be
displayed "on top of the source line". This is incorrect, as they are
currently displayed on the right side of the source line (or on top of
the eBPF instruction, not the source).

This commit fixes the documentation to address these points.

Signed-off-by: Quentin Monnet <quentin@isovalent.com>
Link: https://lore.kernel.org/r/20230405132120.59886-2-quentin@isovalent.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>