Document fieldref combinations and remove the duplicate neq entry

[mostafa]

Summary

This records behavior pySigma already implements.

• fieldref rejects a referenced name that contains wildcards (SigmaFieldReferenceModifier raises "Field references must not contain wildcards").
• fieldref may be followed by contains, startswith, or endswith. Those modifiers set the field reference's starts_with and ends_with flags (contains sets both). A string modifier written first inserts wildcards, which the field reference then rejects, so the string modifier has to come after fieldref.
• fieldref may be combined with neq. In pySigma, neq is SigmaNegateModifier, which negates the detection item. It is not the numeric SigmaNotEqualModifier.
• The Numeric Modifiers section listed neq a second time. That entry is removed. The generic neq entry stays.

Test plan

• mdformat --check specification/sigma-appendix-modifiers.md

[nasbench]

The changes should target the v2.2 branch. I already changed it for you, please resolve the conflcits.