bpf, docs: Clarify PC use in instruction-set.rst

This patch elaborates on the use of PC by expanding the PC acronym,
explaining the units, and the relative position to which the offset
applies.

Signed-off-by: Dave Thaler <dthaler1968@googlemail.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Reviewed-by: David Vernet <void@manifault.com>
Link: https://lore.kernel.org/bpf/20240426231126.5130-1-dthaler1968@gmail.com