detect/byte-jump: Document var usage for nbytes

Issue: 6105

doc/userguide/rules/differences-from-snort.rst

@@ -263,6 +263,16 @@ See :doc:`http-keywords` for all HTTP keywords.
    use ``byte_extract`` and ``byte_test`` to verify that they
    work as expected.
 
+
+``byte_jump`` Keyword
+---------------------
+
+-  Suricata allows a variable name from ``byte_extract`` or
+   ``byte_math`` to be specified for the ``nbytes`` value. The
+   value of ``nbytes`` must adhere to the same constraints
+   as if it were supplied directly in the rule.
+
+
 ``byte_math`` Keyword
 ---------------------
 
@@ -276,7 +286,7 @@ See :doc:`http-keywords` for all HTTP keywords.
    uint32 value. Snort rejects ``rvalue`` values of ``0`` and requires
    values to be between ``[1..max-uint32 value]``.
 
-- Suricata will never match if there's a zero divisor. Division by 0 is undefined.
+-  Suricata will never match if there's a zero divisor. Division by 0 is undefined.
 
 ``byte_test`` Keyword
 ---------------------

doc/userguide/rules/payload-keywords.rst

@@ -517,7 +517,7 @@ The ``byte_jump`` keyword allows for the ability to select a ``<num of bytes>``
 
 Format::
 
-  byte_jump:<num of bytes>, <offset> [, relative][, multiplier <mult_value>] \
+  byte_jump:<num of bytes> | <variable-name>, <offset> [, relative][, multiplier <mult_value>] \
         [, <endian>][, string, <num_type>][, align][, from_beginning][, from_end] \
         [, post_offset <value>][, dce][, bitmask <value>];
 
@@ -525,6 +525,7 @@ Format::
 
 +-----------------------+-----------------------------------------------------------------------+
 | <num of bytes>        | The number of bytes selected from the packet to be converted          |
+|                       | or the name of a byte_extract/byte_math variable.                     |
 +-----------------------+-----------------------------------------------------------------------+
 | <offset>		| Number of bytes into the payload					|
 +-----------------------+-----------------------------------------------------------------------+