eBPF.md: Document BPF_PSEUDO_KFUNC_CALL for the call instruction

When the source register of a call instruction is set to
BPF_PSEUDO_KFUNC_CALL (2), then the call refers to a kernel function.
This is different from a helper function, in that the function is not
specific to BPF programs (but it must be explicitly marked as call-able
from BPF programs).

This is supported in the kernel since commit:
e6ac2450d6de ("bpf: Support bpf program calling kernel function")

Author: qmonnet

eBPF.md

@@ -193,6 +193,7 @@ Opcode         | Mnemonic            | Pseudocode
 0xdd           | jsle dst, src, +off | PC += off if dst <= src (signed)
 0x85 (src = 0) | call imm            | Helper function call
 0x85 (src = 1) | call imm            | BPF function call
+0x85 (src = 2) | call imm            | Kernel function call
 0x95           | exit                | return r0
 
 ### 32-bit