Add a footnote to the "Range patterns" grammar saying when ObsoleteRangePattern is allowed

Author: mattheww

src/patterns.md

@@ -408,7 +408,7 @@ match tuple {
 > _RangePattern_ :\
 >       _InclusiveRangePattern_\
 >    | _HalfOpenRangePattern_\
->    | _ObsoleteRangePattern_
+>    | _ObsoleteRangePattern_[^obsolete-range-edition]
 >
 > _InclusiveRangePattern_ :\
 >       _RangePatternBound_ `..=` _RangePatternBound_
@@ -426,6 +426,8 @@ match tuple {
 > &nbsp;&nbsp; | `-`<sup>?</sup> [FLOAT_LITERAL]\
 > &nbsp;&nbsp; | [_PathInExpression_]\
 > &nbsp;&nbsp; | [_QualifiedPathInExpression_]
+>
+> [^obsolete-range-edition]: Beginning with the 2021 edition, _ObsoleteRangePattern_ is not allowed.
 
 Range patterns match values within the range defined by their bounds. A range pattern may be
 closed or half-open. A range pattern is closed if it has both a lower and an upper bound, and