bpf, docs: Add note about type convention

Add explanation about use of "u64", "u32", etc. as
the type convention used in BPF documentation.
Signed-off-by: Dave Thaler <dthaler@microsoft.com>
Acked-by: David Vernet <void@manifault.com>
Link: https://lore.kernel.org/r/20230127014706.1005-1-dthaler1968@googlemail.comSigned-off-by: Alexei Starovoitov <ast@kernel.org>

Documentation/bpf/instruction-set.rst

@@ -7,6 +7,11 @@ eBPF Instruction Set Specification, v1.0
 
 This document specifies version 1.0 of the eBPF instruction set.
 
+Documentation conventions
+=========================
+
+For brevity, this document uses the type notion "u64", "u32", etc.
+to mean an unsigned integer whose width is the specified number of bits.
 
 Registers and calling convention
 ================================
@@ -164,6 +169,8 @@ the destination register is unchanged whereas for ``BPF_ALU`` the upper
 
   dst = (u32) ((u32) dst + (u32) src)
 
+where '(u32)' indicates that the upper 32 bits are zeroed.
+
 ``BPF_ADD | BPF_X | BPF_ALU64`` means::
 
   dst = dst + src