bpf: fix bpf_skb_pull_data documentation

Fix documentation for bpf_skb_pull_data() helper for
when len == 0.

Fixes: fa15601a ("bpf: add documentation for eBPF helpers (33-41)")
Signed-off-by: Joanne Koong <joannelkoong@gmail.com>
Acked-by: Quentin Monnet <quentin@isovalent.com>
Acked-by: Martin KaFai Lau <kafai@fb.com>
Link: https://lore.kernel.org/r/20220715193800.3940070-1-joannelkoong@gmail.comSigned-off-by: Alexei Starovoitov <ast@kernel.org>

include/uapi/linux/bpf.h

@@ -2361,7 +2361,8 @@ union bpf_attr {
  * 		Pull in non-linear data in case the *skb* is non-linear and not
  * 		all of *len* are part of the linear section. Make *len* bytes
  * 		from *skb* readable and writable. If a zero value is passed for
- * 		*len*, then the whole length of the *skb* is pulled.
+ *		*len*, then all bytes in the linear part of *skb* will be made
+ *		readable and writable.
  *
  * 		This helper is only needed for reading and writing with direct
  * 		packet access.

tools/include/uapi/linux/bpf.h

@@ -2361,7 +2361,8 @@ union bpf_attr {
  * 		Pull in non-linear data in case the *skb* is non-linear and not
  * 		all of *len* are part of the linear section. Make *len* bytes
  * 		from *skb* readable and writable. If a zero value is passed for
- * 		*len*, then the whole length of the *skb* is pulled.
+ *		*len*, then all bytes in the linear part of *skb* will be made
+ *		readable and writable.
  *
  * 		This helper is only needed for reading and writing with direct
  * 		packet access.