bpf, docs: Add introduction for use in the ISA Internet Draft

The proposed intro paragraph text is derived from the first paragraph
of the IETF BPF WG charter at https://datatracker.ietf.org/wg/bpf/about/Signed-off-by: Dave Thaler <dthaler1968@gmail.com>
Acked-by: David Vernet <void@manifault.com>
Link: https://lore.kernel.org/r/20240422190942.24658-1-dthaler1968@gmail.comSigned-off-by: Alexei Starovoitov <ast@kernel.org>

Documentation/bpf/standardization/instruction-set.rst

@@ -5,7 +5,11 @@
 BPF Instruction Set Architecture (ISA)
 ======================================
 
-This document specifies the BPF instruction set architecture (ISA).
+eBPF (which is no longer an acronym for anything), also commonly
+referred to as BPF, is a technology with origins in the Linux kernel
+that can run untrusted programs in a privileged context such as an
+operating system kernel. This document specifies the BPF instruction
+set architecture (ISA).
 
 Documentation conventions
 =========================