bpf, docs: Add docs on extended 64-bit immediate instructions

Add docs on extended 64-bit immediate instructions, including six instructions
previously undocumented.  Include a brief description of maps and variables,
as used by those instructions.

V1 -> V2: rebased on top of latest master

V2 -> V3: addressed comments from Alexei

V3 -> V4: addressed comments from David Vernet

Signed-off-by: Dave Thaler <dthaler@microsoft.com>
Link: https://lore.kernel.org/r/20230326054946.2331-1-dthaler1968@googlemail.com


Signed-off-by: Alexei Starovoitov <ast@kernel.org>