doc/byte_jump: Clarify bitmask operation

Issue: 6693

Clarify how the bitmask value is used for byte_jump

Snort compatibility says:
- The bitmask value is applied to the extracted value before the
  multiplier is applied.
- The result of the bitmask operation is to be right shifted by the
  number of trailing 0's in the bitmask value.

doc/userguide/rules/payload-keywords.rst

@@ -542,6 +542,8 @@ byte_jump
 ---------
 
 The ``byte_jump`` keyword allows for the ability to select a ``<num of bytes>`` from an ``<offset>`` and moves the detection pointer to that position.  Content matches will then be based off the new position.
+The ``bitmask`` value is applied to the extracted value before the multiplier. Additionally, the result of the bitmask operation is
+right-shifted by the number of trailing zeroes in the bitmask value.
 
 Format::