bpf, docs: Correct source of offset for program-local call

The offset to use when calculating the target of a program-local call is
in the instruction's imm field, not its offset field.

Signed-off-by: Will Hawkins <hawkinsw@obs.cr>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Eduard Zingerman <eddyz87@gmail.com>
Acked-by: David Vernet <void@manifault.com>
Link: https://lore.kernel.org/bpf/20230826053258.1860167-1-hawkinsw@obs.cr

Documentation/bpf/standardization/instruction-set.rst

@@ -373,7 +373,7 @@ BPF_JNE   0x5    any  PC += offset if dst != src
 BPF_JSGT  0x6    any  PC += offset if dst > src                    signed
 BPF_JSGE  0x7    any  PC += offset if dst >= src                   signed
 BPF_CALL  0x8    0x0  call helper function by address              see `Helper functions`_
-BPF_CALL  0x8    0x1  call PC += offset                            see `Program-local functions`_
+BPF_CALL  0x8    0x1  call PC += imm                               see `Program-local functions`_
 BPF_CALL  0x8    0x2  call helper function by BTF ID               see `Helper functions`_
 BPF_EXIT  0x9    0x0  return                                       BPF_JMP only
 BPF_JLT   0xa    any  PC += offset if dst < src                    unsigned
@@ -424,8 +424,8 @@ Program-local functions
 ~~~~~~~~~~~~~~~~~~~~~~~
 Program-local functions are functions exposed by the same BPF program as the
 caller, and are referenced by offset from the call instruction, similar to
-``BPF_JA``.  A ``BPF_EXIT`` within the program-local function will return to
-the caller.
+``BPF_JA``.  The offset is encoded in the imm field of the call instruction.
+A ``BPF_EXIT`` within the program-local function will return to the caller.
 
 Load and store instructions
 ===========================